Ridiculous claim by one area of the UK that's hostile (8)
I believe the answer is:
inimical
'hostile' is the definition.
(similar in meaning)
'ridiculous claim by one area of the uk' is the wordplay.
'ridiculous' indicates an anagram.
'by' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'area of the uk' becomes 'NI' (Northern Ireland).
'claim' with letters rearranged gives 'mical'.
'i'+'ni'='ini'
'mical' after 'ini' is 'INIMICAL'.
'that's' acts as a link.
(Other definitions for inimical that I've seen before include "Adverse" , "Hostile, unfavourable" , "Unfriendly or unfavourable" , "Hostile or harmful" , "In claim I am strangely hostile" .)