Girl at a celebration (4)
I believe the answer is:
gala
'celebration' is the definition.
(I know that gala is a type of celebration)
'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 "Prod with the elbow" , "Fête" , "Sports meeting" , "A show of charity" , "Festive entertainment" .)