Many not worried it means the reverse (7)
I believe the answer is:
antonym
'the reverse' is the definition.
(thesaurus)
'many not worried' is the wordplay.
'worried' indicates an anagram.
'many'+'not'='manynot'
'manynot' anagrammed gives 'ANTONYM'.
'it means' acts as a link.
(Other definitions for antonym that I've seen before include "A word's opposite" , "opposing term" , "that means reverse" , "One's import is contrary" , "A word opposite in meaning to another" .)