Unruly bore postures wildly (12)
I believe the answer is:
obstreperous
'unruly' is the definition.
(synonyms)
'bore postures wildly' is the wordplay.
'wildly' indicates anagramming the letters.
'bore'+'postures'='borepostures'
'borepostures' with letters rearranged gives 'OBSTREPEROUS'.
(Other definitions for obstreperous that I've seen before include "Noisily aggressive" , "Noisily defiant" , "Sober posture (anag) - unruly" , "Bolshie" , "Noisy, hard to control" .)