Mouth forming Greek letter (5)
I believe the answer is:
delta
I believe this is a double definition.
'mouth' is the first definition.
(I know that delta is a type of mouth)
'greek letter' is the second definition.
(letter from the Greek alphabet)
'forming' is the link.
(Other definitions for delta that I've seen before include "Estuary area where river has split" , "Daedalus started with this" , "Triangular patch of land at mouth of river" , "Code word for D" , "Letter - where river divides" .)