Left-wing militants rejected a suit jacket from Norma (6)

'left-wing militants' is the definition.
(antifascist activists)

'rejected a suit jacket from norma' is the wordplay.
'rejected' says the letters should be written in reverse.
'suit' becomes 'fit' (synonyms).
'jacket from' says to hollow out the word (remove centre letters) (the letters which form the word\'s case or jacket).
'norma' with its middle removed is 'na'.
'a'+'fit'+'na'='afitna'
'afitna' reversed gives 'ANTIFA'.