Unruly old boy upset superstore (12)
I believe the answer is:
obstreperous
'unruly' is the definition.
('obstreperous' can be a synonym of 'unruly')
'old boy upset superstore' is the wordplay.
'old boy' becomes 'ob'.
'upset' indicates anagramming the letters.
'superstore' anagrammed gives 'streperous'.
'ob'+'streperous'='OBSTREPEROUS'
(Other definitions for obstreperous that I've seen before include "Noisy, hard to control" , "Sober posture (anag) - unruly" , "Noisily defiant" , "Bolshie" , "Noisily aggressive" .)