Prohibition — one sent to outhouse is ostracised (8)
I believe the answer is:
banished
'ostracised' is the definition.
('banish' can be a synonym of 'ostracize')
'prohibition one sent to outhouse' is the wordplay.
'prohibition' becomes 'ban' (ban is a kind of prohibition).
'one' becomes 'i' (Roman numeral).
'sent to' means one lot of letters go next to another.
'outhouse' becomes 'shed' (I've seen this before).
'ban'+'i'+'shed'='BANISHED'
'is' is the link.
(Other definitions for banished that I've seen before include "Condemned to exile" , "Sent into exile" , "Exiled" , "excluded?" .)