Heavy blow for party (4)
I believe the answer is:
bash
I believe this clue is a double definition.
'heavy blow' is the first definition.
(I've seen this before)
'party' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for bash that I've seen before include "Strike violently" , "Smite; a party" , "Batter" , "social occasion" , "Party; beat" .)