Crossword Genius

Longer and shorter periods reversed in final letter (5)

Ross

I believe the answer is:

omega

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'final letter' is the definition.
(I've seen this before)

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for omega that I've seen before include "Greek finalist" , "letter to Greeks" , "Athenian character" , "character from Athens" , "long letter" .)

I've seen this clue in The Independent.
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