Outlaw ultimately overcame nemesis (4)
I believe the answer is:
bane
'nemesis' is the definition.
(synonyms)
'outlaw ultimately overcame' is the wordplay.
'outlaw' becomes 'ban' (banning is a kind of outlawing).
'ultimately' says to take the final letters.
The final letter of 'overcame' is 'e'.
'ban'+'e'='BANE'
(Other definitions for bane that I've seen before include "Blight" , "it causes misery" , "Something causing misery or distress" , "scourge" , "Cause of great distress, rhymes with rule" .)