Old isolationist is Unionist sleeper (7)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

U (abbreviation for Unionist) + KIPPER (kip, as a verb = slang for sleep). Term for someone in favour of Britain’s exit from the EU, as advocated by the UK Independence Party. I’m not sure about “old” because the party still exists, though in much reduced circumstances.