Strike hard; party (4)
I believe the answer is:
bash
I believe this clue is a double definition.
'strike hard' is the first definition.
(I've seen this in another clue)
'party' is the second definition.
(term for a party or celebration)
(Other definitions for bash that I've seen before include "Try - rave-up" , "Party - punch" , "Go" , "crash > party" , "knock" .)