Or brute poses a problem, being unruly (12)
I believe the answer is:
obstreperous
'being unruly' is the definition.
('obstreperous' can be a synonym of 'unruly')
'or brute poses a problem' is the wordplay.
'a problem' indicates anagramming the letters.
'or'+'brute'+'poses'='orbruteposes'
'orbruteposes' with letters rearranged gives 'OBSTREPEROUS'.
(Other definitions for obstreperous that I've seen before include "Noisily aggressive" , "Bolshie" , "Noisily defiant" , "Sober posture (anag) - unruly" , "Disruptive" .)