Wary, cautious (2,4,5)
I believe the answer is:
on ones guard
I believe this is a double definition.
'wary' is the first definition.
(I've seen this in another clue)
'cautious' is the second definition.
(I've seen this before)
(Other definitions for on ones guard that I've seen before include "Alert for possible danger" , "Looking out for danger" , "Cautious" , "Wary" .)