One before Many by eXternal

Answers to ten clues are thematically reduced by a letter. The redundant letters when rearranged suggest a pattern to be highlighted in the finished grid. All other clues contain a misprint in the definition. In clue order the correct letters spell out a relevant quotation. The title suggests the one unique letter to be changed within the grid. Three extra clues without definition reveal answers sharing common ground, this should aid the solver. All enumeration refers to answers, not grid entries.


Across
    1    Thin batteries supplied regularly to doctors for instrument often used with thin (6)
    6    Will's to stir and turn over embraced by his queen intimately (6)
    10    Visit topless musician displaying sweet thickness (8)
    12    In demand, reliable stock to be rotated (7)
    13    Monkey and sloth following goose (6)
    14    Rag's article and column had taste (8)
    16    British crook with the police (4)
    17    Family in recession getting satellite, that's extremely good (7)
    18    A knight's framed by the parson close to king (5)
    20    Scientist from Iceland is said to be very angry (5)
    22    Crafty deer losing head of herd (4)
    24    Lad reverses over his sister and shows signs of dazing (4)
    26    Parts of curvy artistic figures, having removed uniform, receive love (5)
    28    Manfully oppose bidder after bronze (6)
    30    Moping in pain, cramp's starting in punctured side (7)
    31    Arm smashed brat hard (3)
    33    Opal stops volatile ex disappearing with son (8)
    35    Accept wags associated with another one, anonymous and concerning (5)
    36    One set about guarding that foremast (7)
    37    Eastern cops might contain this troublesome teenager (5,3)
    38    Pub on marsh is about one grand (5)
    39    People benefitted from number of Cockneys playing pieces (7)

 Down
    1    Sue has child in the morning, doubled over (4)
    2    Face of America, retired airman's found with a prostitute (7)
    3    Belted out for fan (3,3)
    4    Draper's creations fill boxes with love (8)
    5    Admin trainee's clothing lines unfashionable, including every thong (3,4)
    6    Swiss blusher twice applied to base (4)
    7    Starring in European short film (4)
    8    Soldiers fix plane (5)
    9    Most teak and deal gaining weight over time (7)
    11    Board meeting's leader hides in arranged break (6)
    15    Ken is returning on empty bus (4)
    18    Rank sandwiches seen horribly getting less marge (8)
    19    Popular guide's curtsy (2,5)
    21    Unknown bird in good dog's gob (7)
    22    A scientific individual's code (5)
    23    Marks found after sticky stuff covering single made gramophone parp (4,3)
    25    Grave doubt over drug (6)
    27    Seer at home with pickpocket outside (7)
    29    Room wanting primary colour (5)
    32    One after gross coat (5)
    33    Meals for metalworking types heartily eaten, a ton's shifted (4)
    34    Obscene book taken out for special loans (4)

Extra clues:
Some anger management (6)
I'm about to have tea with the Spanish (7)
Article doctor read with son (7)
To enter this competition, send your entry as an image or in list format, indicating the highlighted entry, to quiz.man@ntlworld.com before the 8th October 2012. The first correct entry drawn from the hat will receive a copy of  one of the Chambers Brewer's Dictionary of Phrase and Fable which has been donated by Chambers.