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