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