Longer and shorter periods reversed in final letter (5)
I believe the answer is:
omega
'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" .)