Extramarital affair (8)
I believe the answer is:
adultery
'extramarital affair' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for adultery that I've seen before include "Moral lapse" , "form of betrayal" , "Marital infidelity - try a duel (anag)" , "Extramarital affair" , "Seventh Commandment infraction" .)