Drunken bore's posture becomes unruly (12)
I believe the answer is:
'unruly' is the definition.
('obstreperous' can be a synonym of 'unruly')
'drunken bore's posture' is the wordplay.
'drunken' is an anagram indicator.
'boresposture' is an anagram of 'OBSTREPEROUS'.
'becomes' is the link.
(Other definitions for obstreperous that I've seen before include "Sober posture (anag) - unruly" , "Noisily aggressive" , "Noisy, hard to control" , "Disruptive" , "Noisily defiant" .)