Slow-witted duke seen dancing (5)
I believe the answer is:
dense
'slow-witted' is the definition.
(I've seen this before)
'duke seen dancing' is the wordplay.
'duke' becomes 'd' (abbreviation).
'dancing' indicates an anagram.
'seen' with letters rearranged gives 'ense'.
'd'+'ense'='DENSE'
(Other definitions for dense that I've seen before include "Closely-spaced" , "Closely compacted" , "Thick as undergrowth may be" , "Closely packed" , "Slow-witted" .)