Greek character, officer turning up as a substitute (5)
I believe the answer is:
locum
'a substitute' is the definition.
(locum is a kind of substitute)
'greek character officer turning up' is the wordplay.
'greek character' becomes 'mu' (letter from the Greek alphabet).
'officer' becomes 'col' (I've seen this before).
'turning up' says the letters should be written in reverse.
'mu'+'col'='mucol'
'mucol' back-to-front is 'LOCUM'.
'as' is the link.
(Other definitions for locum that I've seen before include "A substitute or stand-in doctor" , "Person filling in" , "Doctor's temporary replacement" , "Temporary deputy for doctor" , "Temporary replacement doctor" .)