Stupid duke gets back (4)
I believe the answer is:
daft
'stupid' is the definition.
(thesaurus)
'duke gets back' is the wordplay.
'duke' becomes 'd'.
'gets' says to put letters next to each other.
'back' becomes 'aft' ('aft' is associated in meaning with 'back'**).
'd'+'aft'='DAFT'
(Other definitions for daft that I've seen before include "'Silly, foolish (4)'" , "Informally silly or crazy" , "Bats!" , "Idiotic" , "Half-witted" .)