Playfair Grid by Shackleton

Answers to 1ac, 34ac, 9dn, and 19dn must be playfair encoded prior to entry. A playfair code makes use of a 5x5 square of the 26 letters of the alphabet with I doing duty for both I and J. For example:

The words to be encoded are processed two letters at a time: PE->IA (letters at opposite corners of a rectangle move left or right to the adjacent corners), NT->MO and UP->PD (letters in the same row or column move one place right or down, and end letters are treated cyclically). So PENT-UP-> IAMOPD in this example.

Solvers should locate a relevant 12ac in the grid and highlight 29dn it 10ac the encoding.

Some proper nouns are not in Chambers.

1 High protein food makes them worried about exercise (6)
5 Fiddle with a cobra, but for support, provide an antidote (5)
10 Things that help a serious condition (4)
12 Pious woman covers front of nude with strips of foil (6)
13 Present look that is plain (7)
14 A very corrupt Australian (4)
15 African flies from Japan to Zambia in jet set outing (6)
16 Smart person, over eleven, unable to read (6)
18 Construct bikeway in East by this river, possibly (4)
19 Stress over hemisphere not initially as fraught (11)
22 Terrible bogy monster (4)
24 Ball returned, hitting net - sisters' name now famous (6)
27 Bold knight following in rough apparel (6)
28 Fit of pique, lacking energy? It's a joke (4)
30 Wander around outside where, in France, these foods are often eaten from cartons (7)
31 Besides eating right, I'd like a doughnut (6)
32 What's found on border of lake? University rower, capsized (4)
33 Curved handle of scythe hasn't broken (5)
34 The Amazon, dashing emergently from the Andes, winds across the plains of South America (6)

1 Hit by Prince, a rock legend (5)
2 Unfortunately I've limited right for transference of surplus funds (4)
3 To get pi, do this root operation! (6)
4 Flick this vicious weapon (cane) to strike tutee's backside (11)
6 University marks covered by news releases (6)
7 Already taken inside most sacred parts of temple (5)
8 Is new rise in price to be shown? (7)
9 This plankton (marine) may be digested in sea-monster (6)
11 Diamonds are producing stare (4)
17 Middle Eastern country rejects Christmas, prescribing ban (7)
19 Empty youth have abandoned God (6)
20 Pay (the bill)? Gosh, first pigs might fly! (6, two words))
21 Advance runs with determined enthusiasm (6)
23 Honourable girl's game rhetoric putting down Tory (5)
25 Run after run returned with hard ball (4)
26 Some jockeys parking inside this town? (5)
29 Trees with tonic bark, not date palms (4)

To enter the competition send a solution in list format, the highlighted entry plus an alternative clue to 12 across to before 1st June. The writer of the best clue will receive a copy of  Names, Names, Names by Hugh McEntire.