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' ().
'at' means one lot of letters go next to another.
'gal'+'a'='GALA'
(Other definitions for gala that I've seen before include "sporting event" , "Festivity - celebration" , "Sports meeting" , "Public festivity" , "Festival" .)