Make certain points by river (6)
I believe the answer is:
'make certain' is the definition.
'points by river' is the wordplay.
'points' becomes 'en' (compass points east and north).
'by' means one lot of letters go next to another.
'river' becomes 'sure' (I can't justify this - if you can you should believe this answer much more).
Can you help me to learn more?
(Other definitions for ensure that I've seen before include "Guarantee" , "Make certain, clinch" , "Make certain or safe" , "warrant" , "Protect" .)