Many not worried — it means quite the reverse (7)
I believe the answer is:
antonym
'the reverse' is the definition.
(thesaurus)
'many not worried' is the wordplay.
'worried' is an anagram indicator.
'many'+'not'='manynot'
'manynot' is an anagram of 'ANTONYM'.
'it means quite' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for antonym that I've seen before include "as host is for guest, eg" , "As 'happy' is to 'sad' or 'long' to 'short'" , "opposing term" , "something completely different" , "A word's opposite" .)