Girl at a party (4)
I believe the answer is:
gala
'party' is the definition.
(I know that party can be written as gala)
'girl at a' is the wordplay.
'girl' becomes 'gal' ().
'at' is a charade indicator (letters next to each other).
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "A show of charity" , "Special performance" , "Carnival, fete" , "field day" , "Festive event" .)