Cautious of conflict with an unknown (4)
I believe the answer is:
wary
'cautious' is the definition.
('wary' can be similar in meaning to 'cautious')
'conflict with an unknown' is the wordplay.
'conflict' becomes 'war' (war is a kind of conflict).
'with' means one lot of letters go next to another.
'an unknown' becomes 'y' (algebra).
'war'+'y'='WARY'
'of' acts as a link.
(Other definitions for wary that I've seen before include "Distrustful" , "Careful - cautious" , "Guarded" , "Circumspect" , "Watchful" .)