Britain had one politician detained by Ireland (6)
I believe the answer is:
empire
'britain' is the definition.
The answer and definition can be both locations as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'one politician detained by ireland' is the wordplay.
'one politician' becomes 'MP' (Member of Parliament).
'detained by' is an insertion indicator (I've seen this in other clues).
'ireland' becomes 'eire' (poetic name for Ireland).
'mp' placed into 'eire' is 'EMPIRE'.
'had' acts as a link.
Can you help me to learn more?
(Other definitions for empire that I've seen before include "Over which 23 down rules" , "The - Strikes Back, film" , "Former British rule" , "Large realm" , "Huge domain or group of companies" .)