Start fulminating, being somewhat cunning (6)
I believe the answer is:
artful
'cunning' is the definition.
(similar in meaning)
'start fulminating being somewhat' is the wordplay.
'being somewhat' says the answer is hidden in the clue.
'ARTFUL' can be found hidden inside 'start fulminating'.
(Other definitions for artful that I've seen before include "Crafty or cunning" , "Clever, tricky" , "Dickens boy, an . . . . . . Dodger" , "Clever like Dickens's Dodger" , "Cunningly clever like Dickens Dodger" .)