Helpful act (4,4)
I believe the answer is:
good turn
'helpful act' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for good turn that I've seen before include "one hopes to see in cabaret" , "Helpful deed" , "Favour expected" , "''One . . . . . . . . deserves another'' (4, 4)" , "star performer" .)