Mythical daughter, opponent no longer with us (8)
I believe the answer is:
antigone
'mythical daughter' is the definition.
'opponent no longer with us' is the wordplay.
'opponent' becomes 'anti' (I've seen this before).
'no longer with us' becomes 'gone'.
'anti'+'gone'='ANTIGONE'
(Other definitions for antigone that I've seen before include "The Burial at Thebes is Heaney's translation of Sophocle's (8)" , "Tragedy by Sophocles" , "Jocasta's daughter" , "Greek drama" , "Tragic heroine" .)