Improvise as supplier of a cross, exactly what's needed (4,2)
I believe the answer is:
wing it
'improvise' is the definition.
(I've seen this before)
'supplier of a cross exactly what's needed' is the wordplay.
'supplier of a cross' becomes 'wing' (a winger might provide a cross in football).
'exactly what's needed' becomes 'it'.
'wing'+'it'='WING IT'
'as' acts as a link.
(Other definitions for wing it that I've seen before include "Perform without any preparation" , "Improvise" .)