Miss ace party (4)
I believe the answer is:
gala
'party' is the definition.
(thesaurus)
'miss ace' is the wordplay.
'miss' becomes 'gal' ('gal' can be a synonym of 'miss').
'ace' becomes 'a'.
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "sporting event" , "Festive celebration" , "Special social or sporting occasion" , "Carnival, celebration" , "field day" .)