Smart but impoverished count supplanting duke (11)
I believe the answer is:
intelligent
'smart' is the definition.
('intelligent' can be similar in meaning to 'smart')
'impoverished count supplanting duke' is the wordplay.
I cannot really understand how this works, but
'count' could be 'tell' (I've seen this before) and 'tell' is found in the answer.
This explanation may well be incorrect...
'but' is the link.
(Other definitions for intelligent that I've seen before include "Brainy" , "Capable of reason" , "Able to reason; well-informed" , "Bright, quick of mind" , "Smart" .)