Make certain points by river (6)
I believe the answer is:
ensure
'make certain' is the definition.
(synonyms)
'points by river' is the wordplay.
'points' becomes 'en' (two compass points).
'by' says to put letters next to each other.
'river' becomes 'sure' (I can't justify this - if you can 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 "Guarantee" , "Make certain, clinch" , "Make certain or safe" , "warrant" , "Protect" .)