Stomach muscle’s coming back into operation (6)
I believe the answer is:
accept
'stomach' is the definition.
(I know this)
'muscle's coming back into operation' is the wordplay.
'muscle' becomes 'pec' (I've seen this in other clues).
'coming back' says the letters should be written backwards.
'into' means one lot of letters goes inside another.
'operation' becomes 'act'.
'pec' backwards is 'cep'.
'cep' inserted into 'act' is 'ACCEPT'.
(Other definitions for accept that I've seen before include "Receive something offered" , "Consider or hold as true" , "Believe to be correct; take on" , "Say yes > to trust" , "Take as a gift" .)