Is obliged to interrupt gym period (5)
I believe the answer is:
phase
'period' is the definition.
(phase is a kind of period)
'is obliged to interrupt gym' is the wordplay.
'is obliged' becomes 'has'.
'to interrupt' means one lot of letters goes inside another.
'gym' becomes 'pe'.
'has' going inside 'pe' is 'PHASE'.
(Other definitions for phase that I've seen before include "A particular point in the time of a cycle" , "Truman -, US novelist" , "Distinct period in development" , "Shape into stage" , "Aspect of moon" .)