Gymnast’s costume (7)
I believe the answer is:
leotard
'gymnast's costume' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for leotard that I've seen before include "One-piece garment worn by acrobat" , "Gymnast's tight costume" , "Body suit for dancers" , "Dance/exercise garment" , "Dancers might wear this" .)