Permit admits one at border of county (7)

I believe the answer is:

leitrim

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'county' is the definition.
(county of Ireland)

'permit admits one at border' is the wordplay.
'permit' becomes 'let' (synonyms).
'admits' indicates putting letters inside (to admit is to allow in).
'one' becomes 'i' (Roman numeral).
'at border' becomes 'rim' (I can't explain this - if you can you should give a lot more credence to this answer).
'let' placed around 'i' is 'leit'.
'leit'+'rim'='LEITRIM'

'of' acts as a link.

Can you help me to learn more?

(Other definitions for leitrim that I've seen before include "Limiter of Irish county" , "Could the county rile Tim?" , "County of Ireland" .)

I've seen this clue in The Irish Times.

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