Historical tax for one joining union in 1707 (4)

I believe the answer is:

scot

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'historical tax' is the definition.
'scot' can be an answer for 'tax' (thesaurus). I'm not certain of the 'historical' bit.

I can't explain the rest of the clue.

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(Other definitions for scot that I've seen before include "Rabbie Burns, for instance" , "Native of northern Great Britain" , "Someone from north of the border?" , "Glaswegian perhaps" , "9 [KEIR HARDIE], say" .)

I've seen this clue in The Guardian.

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