Swimmer to show off briefly (4)
I believe the answer is:
swan
'swimmer' is the definition.
(swans can swim)
'show off briefly' is the wordplay.
'show off' becomes 'swank' (synonyms).
'briefly' means to remove the last letter.
'swank' with its final letter taken off is 'SWAN'.
'to' acts as a link.
(Other definitions for swan that I've seen before include "Whooper or trumpeter?" , "Large, long-necked bird" , "black, in 7th" , "Whooper, perhaps" , "Large waterbird" .)