4108, Past and Present by Emkay.

This was a gentle challenge from Emkay. The central square could be filled with the quotation, “A square of text that looks a little blot, (the) text no larger than the limbs of fleas”, from Merlin and Vivien, the sixth poem of Tennyson’s Idylls of the King.  
Thematic entries, clued without definition, gave MESSAGES, MOBILE PHONES, VIVIEN, MERLIN and TENNYSON. When all the answers were entered there remained 7 unused letters. These, plus the missing THE gave “HELLO THERE!”
The puzzle was based on text messages on mobile phones in the present with a comment from the past.

 
Across
1 This can cause skin problems from having resistance suppressed (3)
ORF R (resistance) in OF (from)
4* Herb in hash (8)
MESSAGES SAGE in MESS
9 Short keel missing end of thick metal plate (3)
SEG SKEG less (thic)K
12 Plant lacking colour round about centre of calyx (7)
PARELLE RE (about) + (ca)L(yx) in PALE
13 Attractive changing light in Seychelles (7)
SIGHTLY Anagram LIGHT in SY
14* Bleep him soon in a different way (12, two words)
MOBILE PHONES Anagram BLEEP HIM SOON
15 Light element next to end of Appleton layer (3)
HEN HE (helium) + (appleto)N
16 Smart new set of rules for the web (3)
NIP N (new) + IP
17 Prohibition of the box containing drug revoked (3 + b1)
VETO E in O (of) TV
18 Start changing revised game plan (b8, f3, f8, g1, e7, g6 + 3)
STRATAGEM Anagram START + GAME
22 Benefit from mature years following very advanced nautical training initially (3 + a4, h7, c3, e5)
VANTAGE V + A (advanced) + NT + AGE
23 Lower temperature in Nebraska hospital casualty department (a2, c4, a6 + 3)
NETHER T (temperature) in NE + H + ER
24 Reserve buyer’s option by freezing compartment (3 + f5, c6, d4)
ICE-BOX ICE + BOX 2 definitions
26 Pay following the greatest possible number of customers entering a shop (a7, h5, d6, b5, d1 + 3)
FOOTFALL FOOT (pay) + F (following) + ALL
27 Lamentably bad, Amy haphazardly lost game (3 + h2, a3, b6, g5)
NETBALL
28 City lake beside river (e2, h8, f4 + 3)
LONDON L (lake) + ON + DON
29 Take in loud, active brother (3)
FRA  R in F + A
30 Stammers without hard hesitant sounds (3)
UMS HUMS less H (hard)
32 Road built initially next to deep ravine is perhaps over the border (7)
AIBLINS A1 + B(uilt) + LIN’S
35 United about scrap over city (7)
DETROIT ORT in TIED reversed
37 Old nurse first imbibing drink somewhere in Italy (7)
SALERNO ALE in SRN (nurse) + O (old)
38 High up, alone in orbiter’s furthest position (7)
APOLUNE Anagram UP ALONE
39* State in competition (8)
TENNYSON NY in TENSON
 
Down
1 In this year Portugal initially offered to back up swimmer (4)
OPAH HA (hoc anno) + P + O(ffered) all reversed
2 Rhea’s soul captured by sun-god (5)
RAMEE AME in RE
3 Appearance of new architectural feature (7)
FRONTON FRONT + O’ + N (new)
5 Extra lithium separates in recipe for panacea? (3 + d7, f2, b3)
ELIXIR LI in EX + I + R (recipe)
6 Boil these off bottom of casserole (3 + b4, g7, d3)
SEETHE Anagram THESE + (casserol)E
7 Paths Al reconstructed using this? (3 + g3, e8, h4, b2)
ASPHALT
8 Where at last it’s embraced by elderly venerated person in narcissism (3 + b7, a5, h1, g2)
EGOTISM (wher)E + TIS in GOM (grand old man)
9 Drink, then sag giddily (7)
STENGAH Anagram THEN SAG
10 Man perhaps upset eastern girl (5)
ELSIE ISLE reversed (ref. Isle of Man) + E
11 Where to stretch soft yarn (4)
GYMP GYM + P
17* New European four by four making a comeback (6)
VIVIEN N (new) + E + IV + IV all reversed
19* Scottish blackbird within a small group (6)
MERLIN MERL + IN
20 Somehow a bin retains unfortunate trace of germs (9)
BACTERIAN Anagram A BIN TRACE
21 We shall shortly approach open-armed (9)
WELCOMING WEL(l) + COMING
25 Each supply of food for human consumption (7)
EATABLE EA (each) + TABLE
26 One highway with bend on bend is burdensome (7)
ARDUOUS a RD + U (bend) + O (on) + U + ‘S
29 Trouble in Stornoway with fine volcanic dust (4)
FASH F (fine) + ASH
31 Fix something to eat (4)
STEW 2 definitions
33 Queries about losing English male escorting lady (a1, c8, d8 + 3)
SQUIRE Anagram QUERIES less E
34 Know about angular momentum section of part round ship’s inner foundation (e6, g8, e1 + 3)
KELSON L (angular momentum) + S + O’ in KEN
35 Fasted in a different way? Yes, in time for religious celebration (c1, d2, a8, f6, e4 + 3)
FEAST-DAY Anagram FASTED + AY (yes)
36 Pony design marked on the skin (f7, h3, c5 + 3)
TATTOO 2 definitions