Not many screwed up? Quite the opposite (7)
I believe the answer is:
antonym
'quite the opposite' is the definition.
(a word's antonym is its opposite)
'not many screwed up?' is the wordplay.
'screwed up?' is an anagram indicator (I've seen 'screwed up' mean this).
'not'+'many'='notmany'
'notmany' anagrammed gives 'ANTONYM'.
(Other definitions for antonym that I've seen before include "something completely different" , "A word opposite in meaning to another" , "as host is for guest, eg" , ""Loud" as opposed to "quiet", for example" , "Word that means the opposite of another" .)