Boyfriend admits providing blow (4)
I believe the answer is:
biff
'blow' is the definition.
(I know that biff is a type of blow)
'boyfriend admits providing' is the wordplay.
'boyfriend' becomes 'bf' (common abbreviation for boyfriend).
'admits' means one lot of letters goes inside another.
'providing' becomes 'if' (eg 'providing it's sunny' means 'if it's sunny').
'bf' enclosing 'if' is 'BIFF'.
(Other definitions for biff that I've seen before include "strike" , "Hard blow with fist" , "Punch" .)