Big hitter (12)
I believe the answer is:
sledgehammer
'big hitter' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sledgehammer that I've seen before include "One may be used to break rocks" , "Tool" , "Dealer of heavy blow" , "Unsuitable nutcracker?" , "OTT version of the Nutcracker?" .)