on one's guard (4)
I believe the answer is:
wary
'on one's guard' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for wary that I've seen before include "Watchful" , "Cautious, suspicious" , "Cautious - vigilant" , "Cautious of danger" , "Circumspect" .)