Certain to return during gym study (6)
I believe the answer is:
peruse
'study' is the definition.
(thesaurus)
'certain to return during gym' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to return' is a reversal indicator.
'during' is an insertion indicator.
'gym' becomes 'pe'.
'sure' back-to-front is 'erus'.
'erus' put into 'pe' is 'PERUSE'.
(Other definitions for peruse that I've seen before include "Look over" , "Read attentively" , "Scrutinize" , "Examine at length" , "'Read through, browse (6)'" .)