Greek character in mouth of river (5)
I believe the answer is:
delta
I believe this is a double definition.
'greek character' is the first definition.
(letter from the Greek alphabet)
'mouth of river' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for delta that I've seen before include "Area of distributaries at a river mouth" , "first used by Daedalus" , "Eg, outlet of the Nile" , "Divided river outlet" , "one's often seen at mouth of river" .)