Rook in road on guard (4)
I believe the answer is:
wary
'on guard' is the definition.
(similar in meaning)
'rook in road' is the wordplay.
'rook' becomes 'r' (abbreviation in chess).
'in' is an insertion indicator.
'road' becomes 'way' (I've seen this in other clues).
'r' going into 'way' is 'WARY'.
(Other definitions for wary that I've seen before include "Careful - cautious" , "Cautious - vigilant" , "Cautious and vigilant.17 Add up for infant" , "Cautious of danger" , "Distrustful" .)