Wicked duke keeps tablet hidden (6)
I believe the answer is:
veiled
'hidden' is the definition.
(veiling is a kind of hiding)
'wicked duke keeps tablet' is the wordplay.
'wicked' becomes 'vile' ('vile' can be similar in meaning to 'wicked'**).
'duke' becomes 'd'.
'keeps' is an insertion indicator.
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'vile'+'d'='viled'
'viled' going around 'e' is 'VEILED'.
(Other definitions for veiled that I've seen before include "Partly concealed as with headdress" , "Obscured, hidden" , "Concealed by semi-transparent cloth?" , "Obscured or unclear" , "Thinly covered, face or threat" .)