True lady involved in extramarital sex (8)
I believe the answer is:
adultery
'extramarital sex' is the definition.
(adultery is a kind of extramarital sex)
'true lady involved' is the wordplay.
'involved' indicates an anagram (involved can mean confusing or complex).
'true'+'lady'='truelady'
'truelady' is an anagram of 'ADULTERY'.
'in' acts as a link.
(Other definitions for adultery that I've seen before include "Extramarital sex" , "excluding union involvement?" , "Affairs" , "Unfaithfulness to one's spouse" , "form of betrayal" .)