Physical border, which the sun doesn't cross (3,4)

I believe the answer is:

tan line

'the sun doesn't cross' is the definition.
I know nothing about this answer so I can't judge whether this works.

'physical border' is the wordplay.
'physical' becomes 'tan' (I can't explain this - if you can you should give a lot more credence to this answer).
'border' becomes 'line'.
'tan'+'line'='TAN LINE'

'which' acts as a link.

I've seen this clue in The Guardian.

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