Final letter of the Greek alphabet (5)
I believe the answer is:
omega
'final letter of the greek alphabet' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for omega that I've seen before include "Greek long 'o'" , "Last thing" , "Art of story-telling by body movement" , "Conclusion" , "Greek character symbolizing resistance" .)