Guarantee directions to river (6)
I believe the answer is:
ensure
'guarantee' is the definition.
(I know that guarantee can be written as ensure)
'directions to river' is the wordplay.
'directions' becomes 'en' (east and north).
'to river' becomes 'sure' (I can't explain this - if you can you should believe this answer much more).
'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)" , "Make certain for the future" , "Guarantee" , "Make certain, secure" , "Protect" .)