Party girl with adult (4)
I believe the answer is:
gala
'party' is the definition.
(I know that party can be written as gala)
'girl with adult' is the wordplay.
'girl' becomes 'gal' (synonyms).
'with' is a charade indicator (letters next to each other).
'adult' becomes 'a' (abbreviation).
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "Special performance" , "Fête" , "Special sports event" , "Festive entertainment" , "Social occasion with special performances" .)