Cheating lady true possibly? (8)
I believe the answer is:
adultery
'cheating' is the definition.
(cheating on a partner)
'lady true possibly?' is the wordplay.
'possibly?' indicates anagramming the letters (I've seen 'possibly' mean this).
'lady'+'true'='ladytrue'
'ladytrue' with letters rearranged gives 'ADULTERY'.
(Other definitions for adultery that I've seen before include "Unfaithfulness to one's spouse" , "Moral lapse" , "Extramarital affair" , "Marital infidelity - try a duel (anag)" , "Act of infidelity to spouse" .)