Mature blend of rye? That's cheating! (8)
I believe the answer is:
adultery
'cheating' is the definition.
(cheating on a partner)
'mature blend of rye?' is the wordplay.
'mature' becomes 'adult' (similar in meaning).
'blend of' indicates anagramming the letters (I've seen this in other clues).
'rye' with letters rearranged gives 'ery'.
'adult'+'ery'='ADULTERY'
'that's' acts as a link.
(Other definitions for adultery that I've seen before include "excluding union involvement?" , "form of betrayal" , "Unfaithfulness to one's spouse" , "Marital infidelity - try a duel (anag)" , "cheating" .)