Infidelity upsets true lady (8)
I believe the answer is:
adultery
'infidelity' is the definition.
(thesaurus)
'upsets true lady' is the wordplay.
'upsets' is an anagram indicator.
'true'+'lady'='truelady'
'truelady' with letters rearranged gives 'ADULTERY'.
(Other definitions for adultery that I've seen before include "The act of marital infidelity" , "Extramarital sex" , "Moral lapse" , "Unfaithfulness to one's spouse" , "Extramarital bedroom antics" .)