Unlit playing area has defender crossing line (5-5)
I believe the answer is:
pitch-black
'unlit' is the definition.
(I've seen this before)
'playing area has defender crossing line' is the wordplay.
'playing area' becomes 'pitch' (eg a football pitch).
'has' means one lot of letters go next to another.
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'crossing' is an insertion indicator (to cross can mean to go over or around).
'line' becomes 'l' (used when specifying particular lines from a poem).
'back' enclosing 'l' is 'black'.
'pitch'+'black'='PITCH-BLACK'
(Other definitions for pitch-black that I've seen before include "Completely dark" , "Dreadfully dark" .)