Prisoner not very bright duke cut down (9)
I believe the answer is:
condensed
'cut down' is the definition.
'condensed' can be an answer for 'cut' (thesaurus). I am not certain of the 'down' bit.
'prisoner not very bright duke' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'not very bright' becomes 'dense' (I've seen this before).
'duke' becomes 'd' (abbreviation).
'con'+'dense'+'d'='CONDENSED'
Can you help me to learn more?
(Other definitions for condensed that I've seen before include "cut down to size?" , "being thicker" , "Abridged" , "Changed from gas to liquid" .)