Not a drunken follower of Falstaff but the opposite (7)
I believe the answer is:
antonym
'the opposite' is the definition.
('antonym' can be a synonym of 'opposite')
'not a drunken follower of falstaff' is the wordplay.
'drunken' is an anagram indicator.
'follower of falstaff' becomes 'nym' (I've seen this before).
'not'+'a'='nota'
'nota' anagrammed gives 'anto'.
'anto'+'nym'='ANTONYM'
'but' acts as a link.
(Other definitions for antonym that I've seen before include "Word that means the opposite of another" , "opposing description" , "A word opposite in meaning to another" , "opposing term" , "that means reverse" .)