Line adopted by champion chess player (5)
I believe the answer is:
black
'chess player' is the definition.
(I've seen this before)
'line adopted by champion' is the wordplay.
'line' becomes 'l'.
'adopted by' means one lot of letters goes inside another (I've seen this in other clues).
'champion' becomes 'back' (I've seen this in other clues**).
'l' placed within 'back' is 'BLACK'.
(Other definitions for black that I've seen before include "Gloomy" , "Sort of guard, panther, jack or pudding..." , "Chess player" , "Like clubs and spades" , "Desperate" .)