A cherry tree next to river at its highest point (7)
I believe the answer is:
apogean
'at its highest point' is the definition.
'a cherry tree next to river' is the wordplay.
'cherry tree' becomes 'gean' (gean is a kind of cherry tree).
'next to' says to put letters next to each other.
'river' becomes 'po' (Po is an example).
'gean' put after 'po' is 'pogean'.
'a'+'pogean'='APOGEAN'
(Other definitions for apogean that I've seen before include "reaching a climax" , "at highest level" , "at the top?" , "at the furthest point" .)