Ascertain directions to river (6)
I believe the answer is:
ensure
'ascertain' is the definition.
(I know that ascertain can be written as ensure)
'directions to river' is the wordplay.
'directions' becomes 'en' (east and north).
'to river' becomes 'sure' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'en'+'sure'='ENSURE'
Can you help me to learn more?
(Other definitions for ensure that I've seen before include "Make certain (something will occur)" , "See run (anag)" , "Make certain, confirm" , "Make certain, clinch" , "Make certain that (something occurs)" .)