Girl at a swimming contest (4)
I believe the answer is:
gala
'swimming contest' is the definition.
(as in a swimming gala)
'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 festive occasion, perhaps of sporting nature" , "A show of charity" , "Prod with the elbow" , "fair" , "Carnival, fete" .)