Joke, say, about duke unable to speak (6)
I believe the answer is:
gagged
'unable to speak' is the definition.
(someone wearing a gag cannot speak)
'joke say about duke' is the wordplay.
'joke' becomes 'gag' (a gag is a joke or witticism).
'say' becomes 'eg' (both can mean 'for example').
'about' says the letters should be written backwards.
'duke' becomes 'd' (abbreviation).
'eg' back-to-front is 'ge'.
'gag'+'ge'+'d'='GAGGED'
(Other definitions for gagged that I've seen before include "Struggled for breath or prevented from speaking out" , "Silenced" , "Muffled or retched" , "Not allowed to speak out" .)