Letter; river mouth (5)
I believe the answer is:
delta
This clue is a double definition.
'letter' is the first definition.
(Greek letter)
'river mouth' is the second definition.
(I've seen this in another clue)
(Other definitions for delta that I've seen before include "Mouth of, eg, the Nile" , "Fourth Greek letter? - area at river mouth" , "Triangular patch of land at mouth of river" , "Greek character" , "Silted estuary" .)