Figure of Greek character, one practising medicine (5)
I believe the answer is:
rhomb
'figure' is the definition.
(geometric figure)
'greek character one practising medicine' is the wordplay.
'greek character' becomes 'rho' (letter from the Greek alphabet).
'one practising medicine' becomes 'MB' (Bachelor of Medicine).
'rho'+'mb'='RHOMB'
'of' acts as a link.
(Other definitions for rhomb that I've seen before include "Figure of four equal sides" , "Figure on plane" , "Four-sided figure" , "Crystal shape" , "Equilateral parallelogram" .)