Official set off to battle with duke (7)
I believe the answer is:
steward
'official' is the definition.
(thesaurus)
'set off to battle with duke' is the wordplay.
'off' indicates anagramming the letters.
'to battle' becomes 'war' (I've seen this before**).
'with' is a charade indicator (letters next to each other).
'duke' becomes 'd'.
'set' with letters rearranged gives 'ste'.
'ste'+'war'+'d'='STEWARD'
(Other definitions for steward that I've seen before include "bailiff" , "Official at event" , "Aircraft attendant" , "Airline host" , "Racing official" .)