Cautious course across river (4)
I believe the answer is:
wary
'cautious' is the definition.
('wary' can be similar in meaning to 'cautious')
'course across river' is the wordplay.
'course' becomes 'way' (way is a kind of course**).
'across' means one lot of letters goes inside another.
'river' becomes 'r'.
'way' enclosing 'r' is 'WARY'.
(Other definitions for wary that I've seen before include "Careful - cautious" , "Guarded" , "Distrustful" , "Circumspect" , "Watchful" .)