Swimmer in short dress (3)
I believe the answer is:
'swimmer' is the definition.
(gar is a kind of aquatic vertebrate)
'short dress' is the wordplay.
'short' means to remove the last letter.
'dress' becomes 'garb' ('garb' can be a synonym of 'dress').
'garb' with its final letter removed is 'GAR'.
'in' acts as a link.
(Other definitions for gar that I've seen before include "A fish; cloth scrap (reversed)" , "swimmer" , "Type of fish" , "something fishy here" , "Pike-like fish" .)