Mouth forming Greek letter (5)
I believe the answer is:
delta
I believe this clue is a double definition.
'mouth' is the first definition.
(I've seen this before)
'greek letter' is the second definition.
(letter from the Greek alphabet)
'forming' acts as a 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" .)