A duke in cast, underhand (5)
I believe the answer is:
shady
'underhand' is the definition.
(I've seen this before)
'a duke in cast' is the wordplay.
'duke' becomes 'd'.
'in' means one lot of letters goes inside another.
'cast' becomes 'shy' (I've seen this before**).
'a'+'d'='ad'
'ad' going within 'shy' is 'SHADY'.
(Other definitions for shady that I've seen before include "Disreputable - umbrageous" , "Of doubtful legality" , "Of questionable morality out of the sunlight" , "Questionably legal" , "Disreputable -- out of the sun" .)