Sign of summer beginning in Santorini (5)

I believe the answer is:

sigma

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'sign' is the definition.
(sigma is a kind of sign)

I cannot understand how the rest of the clue works.

(Other definitions for sigma that I've seen before include "Greek letter signifying sum in maths" , "character in Corfu" , "Foreign character" , "Allowing no relaxation" , "Letter from Greece" .)

I've seen this clue in The Independent.

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