Old Greek author sees river water rising (5)
I believe the answer is:
aesop
'old greek author' is the definition.
(ancient Greek author of fables)
'river water rising' is the wordplay.
'river' becomes 'Po' (Italian river).
'water' becomes 'sea' (the sea is water).
'rising' is a reversal indicator (going up in a down clue).
'po'+'sea'='posea'
'posea' backwards is 'AESOP'.
'sees' acts as a link.
(Other definitions for aesop that I've seen before include "classic author" , "Ancient Greek fabulist" , "story teller" , "Greek author of fables (c 620-560 BC)" , "Greek storyteller" .)