Not many turning out to be the opposite (7)
I believe the answer is:
antonym
'the opposite' is the definition.
('antonym' can be a synonym of 'opposite')
'not many turning out' is the wordplay.
'turning out' indicates an anagram.
'not'+'many'='notmany'
'notmany' anagrammed gives 'ANTONYM'.
'to be' acts as a link.
(Other definitions for antonym that I've seen before include "Nick's could be free" , "One's import is contrary" , "something completely different" , "A word's opposite" , "as host is for guest, eg" .)