Part of a set at the gym (3)
I believe the answer is:
rep
'part of a set at the gym' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rep that I've seen before include "Repertory theatre" , "Material for curtains" , "Fabric with a ribbed surface" , "Sort of theatre" , "what a draper usually sells" .)