Girl at a swimming competition (4)
I believe the answer is:
gala
'swimming competition' is the definition.
(I've seen this before)
'girl at a' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl'**).
'at' is a charade indicator (letters next to each other).
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "sporting event" , "Festivity - celebration" , "Sports meeting" , "Public festivity" , "Festival" .)