Greek character, doctor, in shape (5)
I believe the answer is:
rhomb
'shape' is the definition.
(rhomb is a kind of shape)
'greek character doctor' is the wordplay.
'greek character' becomes 'rho' (letter from the Greek alphabet**).
'doctor' becomes 'mb' (Bachelor of Medicine, Medicinae Baccalaureus).
'rho'+'mb'='RHOMB'
'in' is the link.
(Other definitions for rhomb that I've seen before include "Diamond shape" , "Figure of four equal sides" , "Crystal shape" , "Quadrilateral with four equal sides" , "Figure on plane" .)