Partisan fighting in autumn, briefly (9)

'partisan' is the definition.
(thesaurus)

'fighting in autumn briefly' is the wordplay.
'fighting' becomes 'action' (as in military action).
'in' means one lot of letters goes inside another.
'autumn' becomes 'fall' (synonyms).
'briefly' means to remove the last letter.
'fall' with its last letter taken away is 'fal'.
'action' placed within 'fal' is 'FACTIONAL'.