Smashing success (7)
(Another definition for homerun that I've seen is " big hit in America".)
'smashing success' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for hit that I've seen before include "Popular song; strike" , "skier, perhaps" , "Of a play, a smashing success" , "recording in the charts" , "Strike, clout" .)