Letter removing tons from disgrace (5)

I believe the answer is:

sigma

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

I don't understand how the remainder of the clue works.

(Other definitions for sigma that I've seen before include "Greek character" , "Greek letter signifying sum in maths" , "letter to Greeks" , "Letter from Greece" , "character in Athens" .)

I've seen this clue in The Times.

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