Greek character central to 'Medea'? (5)

I believe the answer is:

delta

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'greek character' is the definition.
(letter from the Greek alphabet)

I can't explain the remainder of the clue.

(Other definitions for delta that I've seen before include "Alluvial river mouth" , "Area of distributaries at a river mouth" , "Spreading river mouth" , "Multiple estuary" , "Triangular patch of land at mouth of river" .)

I've seen this clue in The Telegraph.

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