Unintelligent study with vacuous style (5)
I believe the answer is:
dense
'unintelligent' is the definition.
(I've seen this in another clue)
'study with vacuous style' is the wordplay.
'study' becomes 'den' (term for a study or office).
'with' means one lot of letters go next to another.
'vacuous' means to remove the middle letters.
'style' with its middle taken out is 'se'.
'den'+'se'='DENSE'
(Other definitions for dense that I've seen before include "Heavy" , "Not very bright" , "Thick, impenetrable" , "Obtuse - impenetrable" , "Close-knit" .)