Mature medic returned for last in series (5)
I believe the answer is:
omega
'last in series' is the definition.
(last letter of the Greek alphabet)
'mature medic returned' is the wordplay.
'mature' becomes 'age' (synonyms).
'medic' becomes 'mo' (medical officer).
'returned' says the letters should be written in reverse.
'age'+'mo'='agemo'
'agemo' in reverse letter order is 'OMEGA'.
'for' acts as a link.
(Other definitions for omega that I've seen before include "Art of story-telling by body movement" , "character in Greek literature?" , "Last letter of the Greek alphabet" , "End of sequence" , "Last (Gk. letter)" .)