Letter from queen ignored by mad hatter (5)

I believe the answer is:

theta

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'letter' is the definition.
(Greek letter)

I don't understand the remainder of the clue.

(Other definitions for theta that I've seen before include "character in Marathon" , "8th letter of the Greek alphabet" , "Letter to the Corinthians" , "letter to Greeks" , "Eighth letter of Greek alphabet" .)

I've seen this clue in The Times.

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