Devil doctor in his study (5)
I believe the answer is:
demon
'devil' is the definition.
(I know that devil can be written as demon)
'doctor in his study' is the wordplay.
I cannot really see how this works, but
'doctor' could be 'mo' (medical officer) and 'mo' is present in the answer.
'study' could be 'den' (term for a study or office) and 'den' is found within the remaining letters.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Evildoer" , "Evil spirit, devil" , "Genius" , "Fanatic; devil" , "malignant spirit" .)