Last character in repechage, mostly upset (5)
I believe the answer is:
omega
'last character' is the definition.
(Greek alphabet)
'in repechage mostly upset' is the wordplay.
'in' indicates the answer is hidden within the clue.
'upset' says the letters should be written in reverse.
'agemo' is hidden in the letters of 'repechage mostly'.
'agemo' reversed gives 'OMEGA'.
(Other definitions for omega that I've seen before include "Final Greek letter" , "letter to Greeks" , "16 letter" , "Greek character symbolizing resistance" , "completes a series" .)