Grown-up rye recipe is cheating (8)
I believe the answer is:
adultery
'is cheating' is the definition.
(cheating on a partner)
'grown-up rye recipe' is the wordplay.
'grown-up' becomes 'adult' (informal term for an adult).
'recipe' indicates anagramming the letters.
'rye' anagrammed gives 'ery'.
'adult'+'ery'='ADULTERY'
(Other definitions for adultery that I've seen before include "Moral lapse" , "Marital infidelity - try a duel (anag)" , "Act of infidelity to spouse" , "Seventh Commandment infraction" , "The act of being unfaithful to spouse (8)" .)