Dirty footballer crossing line (5)
I believe the answer is:
Here is my best explanation:
'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'.