Desperate defender covering line (5)
I believe the answer is:
black
'desperate' is the definition.
(I've seen this before)
'defender covering line' is the wordplay.
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'covering' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'back' going around 'l' is 'BLACK'.
(Other definitions for black that I've seen before include "Can precede art, box, eye" , "Uncommon H [HORSE]" , "What's written on in the classroom" , "Very darkest colour" , "second player on board" .)