Gym has inbuilt stage (5)
I believe the answer is:
phase
'stage' is the definition.
(both can mean a part or period of something)
'gym has inbuilt' is the wordplay.
'gym' becomes 'pe' (short for Physical Education).
'inbuilt' means one lot of letters goes inside another.
'pe' going around 'has' is 'PHASE'.
(Other definitions for phase that I've seen before include "Shape into stage" , "Truman -, US novelist" , "Period within process" , "Shape (anag.)" , "Distinct stage" .)