A successful blow? (3)
I believe the answer is:
hit
I believe this clue is a double definition.
'a successful' is the first definition.
(I've seen this in another clue)
'blow?' is the second definition.
(I know that blow can be written as hit)
(Other definitions for hit that I've seen before include "Strike, clout" , "Murder" , "turkey? No" , "Popular song; strike" , "Of a play, a smashing success" .)