Seriously by Chalicea

Each of three pieces of advice should be followed twice in order to complete the grid. Clues lead to the solution before the advice is followed. Extra letters in the word-play of down clues (in addition to those needed for the solution) tell solvers where they can look for instructions. The author of the advice should be highlighted. ODQ and Chambers (2011) are recommended.

    2    Strangely overall was centrally pitted (8)
    8    Sullen person in acrobatic troupe (3)
    10    Urge old leaderless clan from the north in Scotland (6)
    11    Understand, comprehending revolutionary city gathering (6)
    12    Not liquefied, manganese dioxide at first treated with elute (8)
    14    Animal (cold-blooded one) went astray (4)
    15    Strong beast's wool separated by combing in reverse (4)
    16    Music to silence Gambian ethnic group (6)
    17    One who claims rights without pretence before Queen (7)
    22    Net English and Frenchwomen handled originally (6)
    23    Military operation is repelled for example with energy (5)
    24    Infection? It isn't we are told (5)
    27    Mac's belched! The old parasite, after spitting initially (6)
    28    Involuntarily trembling, turned white, nursing start of sickness and infection (7)
    31    Advance payment to produce an effect and work up (6)
    34    Idiom Channel Island endlessly uses (4)
    35    Broad-winged wader receding in accessibility (4)
    36    Long-bodied swimmer's hard manner to pursue (8)
    37    Spenser's homeland? Name adopted by early Briton off and on (6)
    38    Holy writing of branches of learning Anglo-Saxon recalled (6)
    39    Just a little way off in time - intermittently (3)
    40    Strategic river-crossing of returning highway (8)
    1    Norse God of winter in the region once believed to be furthest north (5)
    3    Shelter bits of marketed nutritious American duckweed (5)
    4    Small duck (English) might have been evil for Shakespeare! (4)
    5    In a state of restless activity scrambling to the phone endlessly (8, 3 words)
    6    Outer garments of female animal turning up within Asia in disarray (6, 2 words)
    7    Utterly defeating atrocious government-run riot (4)
    8    Deplorably repeats obsolete printing activities (6)
    9    Carelessly drew claret, perhaps (7)
    13    Finally fertilise old acre from time to time, one of geological epoch (6)
    18    Cut short service for religious celebration (4)
    19    Monarchist's abnormally costly air (8)
    20    Wrongly placed warped mortises not initially offset (6)
    21    Advanced S-shaped architectural moulding's off the straight in Balmoral (4)
    22    Forced assent that is periodical (7)
    25    Assimilate type of computer game with comprehensive authoritative book (no lecturer) (6)
    26    Negative argument about following school of industrial arts and music (6)
    29    Enraged military unionists let up (5)
    30    Defer proper plan (5)
    32    This expression of incredulity's so Scottish arising with uncertainty (4, 2 words)
    33    Woman in Germany, France, Austria and America (4)
To enter this competition, send your entry as an image or in list format indicating the highlighted entry,  to  before the 8th June 2015. The first correct entry drawn from the hat will receive a book from the Chambers range which has been donated by Chambers.