Argue casually about small brook with a partisan (9)
I believe the answer is:
guerrilla
'partisan' is the definition.
(similar in meaning)
'argue casually about small brook' is the wordplay.
'casually' is an anagram indicator (I've seen this in other clues).
'about' means one lot of letters goes inside another.
'small brook' becomes 'rill' (I've seen this before).
'argue' is an anagram of 'guera'.
'guera' enclosing 'rill' is 'GUERRILLA'.
'with a' is the link.
(Other definitions for guerrilla that I've seen before include "Irregular soldier" , "Combatant" , "Partisan" , "Member of an irregular armed force" , "freedom fighter?" .)