Doctor involved in study of genius (5)
I believe the answer is:
demon
'genius' is the definition.
(I've seen this before)
'doctor involved in study' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'involved in' indicates putting letters inside.
'study' becomes 'den' (I've seen this before).
'mo' placed within 'den' is 'DEMON'.
'of' acts as a link.
(Other definitions for demon that I've seen before include "Evildoer" , "Skilful person" , "Monster" , "malignant spirit" , "Devil - master performer" .)