Plain’s high point with river feeding reservoir (7)
I believe the answer is:
upfront
'river feeding reservoir' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'plain's high point' is the wordplay.
'plain's high' becomes 'up' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'point' becomes 'front' (front is a kind of point).
'up'+'front'='UPFRONT'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for upfront that I've seen before include "Open - paid at start of transaction" , "Direct" , "(Of payment) in advance" , "Bold, frank (colloq.)" , "Informally, straightforward or frank" .)