Guarded route across river (4)
I believe the answer is:
wary
'guarded' is the definition.
(thesaurus)
'route across river' is the wordplay.
'route' becomes 'way' (way is a kind of route**).
'across' indicates putting letters inside.
'river' becomes 'r'.
'way' going around 'r' is 'WARY'.
(Other definitions for wary that I've seen before include "Cautious - vigilant" , "Cautious and vigilant.17 Add up for infant" , "Cautious of danger" , "Cautious, suspicious" , "Displaying caution" .)