Mix online and so spread the North-South divide (5-5,4)

I believe the answer is:

mason-dixon line

'north-south divide' is the definition.
(I've seen this before)

'mix online and so spread' is the wordplay.
'spread' is an anagram indicator.
'mix'+'online'+'and'+'so'='mixonlineandso'
'mixonlineandso' is an anagram of 'MASON-DIXON LINE'.

'the' acts as a link.

(Another definition for mason-dixon line that I've seen is " state boundary".)

I've seen this clue in The Guardian.

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