To work out, I’d bend muscles at regular intervals (6)
I believe the answer is:
deduce
'to work out' is the definition.
(thesaurus)
'i'd bend muscles at regular intervals' is the wordplay.
'at regular intervals' indicates alternate letters.
'id'+'bend'+'muscles'='idbendmuscles'
The alternate letters of 'idbendmuscles' are 'DEDUCE'.
(Other definitions for deduce that I've seen before include "Draw conclusion" , "Reason through logic" , "Infer from the evidence" , "Infer by reasoning" , "Draw conclusions" .)