Mouth a letter (5)

I believe the answer is:

delta

I believe this is a double definition.

'mouth' is the first definition.
(I've seen this before)

'a letter' is the second definition.
(Greek letter)

(Other definitions for delta that I've seen before include "Daedalus started with this" , "Eg, outlet of the Nile" , "River mouth in the Greek alphabet" , "The Greek D (5)" , "Fourth letter of the ancient Greek alphabet: river feature" .)

I've seen this clue in The Irish News.

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