Region once seeing prejudice mostly about old border (7)
I believe the answer is:
bohemia
'region once' is the definition.
(historical European region)
'prejudice mostly about old border' is the wordplay.
'prejudice' becomes 'bias' ('bias' can be a synonym of 'prejudice').
'mostly' means to remove the last letter.
'about' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'border' becomes 'hem' (I've seen this before**).
'bias' with its final letter removed is 'bia'.
'o'+'hem'='ohem'
'bia' enclosing 'ohem' is 'BOHEMIA'.
'seeing' is the link.
(Other definitions for bohemia that I've seen before include "Czech region" , "District for unconventional people - former European kingdom" , "province of Europe" , "Old country" , "Part of the Czech Republic" .)