Heartless Duke professor's not so nice look (9)
I believe the answer is:
demeanour
'look' is the definition.
(I've seen this before)
'heartless duke professor's not so nice' is the wordplay.
I cannot really understand how this works, but
'duke' could be 'd' (abbreviation) and 'd' is found within the answer.
'not' could be 'no' (negative) and 'no' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for demeanour that I've seen before include "Manner of behaving towards others" , "Outward bearing" , "Enamoured of the way a person behaves" , "Appearance and attitude" , "Look" .)