On a river, the fertile area missing a high point (10)
I believe the answer is:
apotheosis
'point' is the definition.
(I have seen 'Crowning point ' mean 'apotheosis' so perhaps 'point' could also mean 'apotheosis')
'on a river the fertile area' is the wordplay.
'on' says to put letters next to each other.
'river' becomes 'Po' (Italian river).
'fertile area' becomes 'osis' (I can't justify this - if you can you should believe this answer much more).
'a'+'po' is 'apo'.
'apo'+'the'+'osis'='APOTHEOSIS'
'missing a high' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for apotheosis that I've seen before include "ideal" , "crowning moment" , "Crowning point" , "Highest point of development" , "Deification" .)