18 down
British
PM
doesn’t
finish
campaign
(6)
British PM doesn’t finish campaign (6)
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The definition is at the end of the clue.
How the clue works
Look at campaign at the end — that's your definition. British gives you B, then PM gives you ATTLEE (Clement Attlee), which doesn't finish, so you lop off the final E to leave ATTLE, and B plus ATTLE gets you there.