Dirty footballer crossing line (5)
I believe the answer is:
black
'dirty' is the definition.
(I've seen this before)
'footballer crossing line' is the wordplay.
'footballer' becomes 'back' (back is a kind of footballer**).
'crossing' indicates putting letters inside.
'line' becomes 'l'.
'back' enclosing 'l' is 'BLACK'.
(Other definitions for black that I've seen before include "Colour of coal" , "Colour of jet" , "It produces illegally undeclared income" , "Dirty" , "Boycott" .)