Girl at a party (4)
I believe the answer is:
gala
'party' is the definition.
(I've seen this in another clue)
'girl at a' is the wordplay.
'girl' becomes 'gal' ().
'at' means one lot of letters go next to another.
'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" .)