An opponent, possibly of note (3)
I believe the answer is:
foe
'an opponent' is the definition.
(I've seen this in another clue)
'possibly of note' is the wordplay.
'possibly' is an anagram indicator.
'note' becomes 'e' (musical note).
'of' anagrammed gives 'fo'.
'fo'+'e'='FOE'
(Other definitions for foe that I've seen before include "Opposition" , "Opponent" , "Someone opposing" , "Am archaic enemy" , "Evil rival" .)