Border of Antrim? (3)

I believe the answer is:

rim

I'm a little stuck... Click here to teach me more about this clue!

'border' is the definition.
(both can mean an edge)

'antrim?' is the wordplay.
'RIM' is hidden in amongst the letters.
But, I'm not sure how the hidden word is indicated.

'of' is the link.

Can you help me to learn more?

(Other definitions for rim that I've seen before include "Edge - lip" , "Circumference" , "'Brim, lip (3)'" , "An edge, canyon or cup" , "Edge or margin" .)

I've seen this clue in the Evening Standard.

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