Party's not initially serious (4)
I believe the answer is:
rave
'party's' is the definition.
(I've seen this before)
'not initially serious' is the wordplay.
'not initially' means to remove the first letter.
'serious' becomes 'grave' (synonyms).
'grave' with its initial letter removed is 'RAVE'.
(Other definitions for rave that I've seen before include "Modern youth dance - sounds hysterical" , "Party with electronic dance music - be wild" , "Illegal party; talk madly" , "Talk incoherently" , "Rant - noisy, all-night party" .)