New enemy's not unknown and is an unbeatable opponent (7)
I believe the answer is:
nemesis
'an unbeatable opponent' is the definition.
(I've seen this in another clue)
'new enemy's not unknown and is' is the wordplay.
'new' indicates an anagram.
'not' suggests deleting specific letters.
'unknown' becomes 'y' (algebra).
'and' says to put letters next to each other.
'enemys' with 'y' removed is 'enems'.
'enems' is an anagram of 'nemes'.
'nemes'+'is'='NEMESIS'
(Other definitions for nemesis that I've seen before include "curse" , "Retribution personified" , "Retributive justice" , "enemy" , "causes downfall" .)