Infidelity hurt true lady (8)
I believe the answer is:
adultery
'infidelity' is the definition.
(I've seen this before)
'hurt true lady' is the wordplay.
'hurt' indicates anagramming the letters.
'true'+'lady'='truelady'
'truelady' with letters rearranged gives 'ADULTERY'.
(Other definitions for adultery that I've seen before include "excluding union involvement?" , "Affairs" , "Seventh Commandment infraction" , "Extramarital sex" , "Betrayal of a partner" .)