Cautious fighting against unknown quantity (4)
I believe the answer is:
wary
'cautious' is the definition.
('wary' can be similar in meaning to 'cautious')
'fighting against unknown quantity' is the wordplay.
'fighting' becomes 'war'.
'against' means one lot of letters go next to another.
'unknown quantity' becomes 'y' (algebra).
'war'+'y'='WARY'
(Other definitions for wary that I've seen before include "Cautious of danger" , "Displaying caution" , "Circumspect" , "James Bond villain" , "Distrustful" .)