Significant ruin besetting Scots sweetheart (5)

I believe the answer is:

major

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'significant' is the definition.
(I've seen this in another clue)

'ruin besetting scots sweetheart' is the wordplay.
'ruin' becomes 'mar' (I've seen this before).
'besetting' indicates putting letters inside.
'scots sweetheart' becomes 'jo' (Scottish term for darling or sweetheart. I am not sure about the 'scots' bit.).
'mar' going around 'jo' is 'MAJOR'.

Can you help me to learn more?

(Other definitions for major that I've seen before include "Principal" , "Leading - ex-Prime Minister" , "Important; old Animal Farm boar" , "Conservative PM" , "Large" .)

I've seen this clue in The Times.

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