No servant upset duke? Brilliant! (7)
I believe the answer is:
diamond
'brilliant' is the definition.
(thesaurus)
'no servant upset duke?' is the wordplay.
'servant' becomes 'maid' (maid is a kind of servant).
'upset' says the letters should be written backwards.
'duke?' becomes 'd' (abbreviation).
'no'+'maid'='nomaid'
'nomaid' reversed gives 'diamon'.
'diamon'+'d'='DIAMOND'
(Other definitions for diamond that I've seen before include "Birthstone for April" , "Very hard gem" , "Sixtieth" , "nickname of US State" , "Suit-baseball field - gem" .)