Honourable silence socially acceptable in part of Japan (6)
I believe the answer is:
honshu
'part of japan' is the definition.
('Honshu' is part of 'japan')
'honourable silence socially acceptable' is the wordplay.
'honourable' becomes 'hon' (as in Rt Hon, Right Honourable).
'silence' becomes 'sh' ('sh' is said to tell someone to be silent).
'socially acceptable' becomes 'U' (as in U and non-U language).
'hon'+'sh'+'u'='HONSHU'
'in' acts as a link.
(Other definitions for honshu that I've seen before include "Largest of the four main islands of Japan" , "The largest island unit of Japan" , "Island regarded as the Japanese mainland" , "Largest island of Japanese mainland" , "part of Japan" .)