Greek character's mouth (5)
I believe the answer is:
delta
I believe this clue is a double definition.
'greek character's' is the first definition.
(letter from the Greek alphabet)
'mouth' is the second definition.
(I've seen this before)
(Other definitions for delta that I've seen before include "Triangular patch of land at mouth of river" , "Roughly triangular area of alluvial deposits" , "Branching river mouth" , "Area of distributaries in a river basin" , "Wide silted river mouth" .)