Drunken bore's posture becomes unruly (12)
I believe the answer is:
obstreperous
'unruly' is the definition.
('obstreperous' can be a synonym of 'unruly')
'drunken bore's posture' is the wordplay.
'drunken' is an anagram indicator.
'bores'+'posture'='boresposture'
'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" .)