Girl at a celebration (4)
'celebration' is the definition.
(gala is a kind of celebration)
'girl at a' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'at' means one lot of letters go next to another.
(Other definitions for gala that I've seen before include "Prod with the elbow", "Fête", "Sports meeting", "A show of charity", "Festive entertainment".)