The letter D has three corners (5)

I believe the answer is:

delta

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

I don't understand the rest of the clue.

(Other definitions for delta that I've seen before include "Wing shape; Greek D" , "The Greek D (5)" , "Estuary area where river has split" , "Comms code for 'd'" , "The fourth Greek letter found at river's mouth" .)

I've seen this clue in the Evening Standard.

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