Scottish proprietary line in cooking fat (5)

I believe the answer is:

laird

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'scottish proprietary' is the definition.
'laird' can be an answer for 'proprietary' (thesaurus). I am not certain of the 'scottish' bit.

'line in cooking fat' is the wordplay.
'line' becomes 'air' (synonyms).
'in' means one lot of letters goes inside another.
'cooking fat' becomes 'ld' (I am not sure about this - if you are sure you should believe this answer much more).
'air' inserted into 'ld' is 'LAIRD'.

Can you help me to learn more?

(Other definitions for laird that I've seen before include "Proprietor" , "Estate landowner" , "Scottish estate-owner" , "One's landed in Scotland" , "Scots estate owner" .)

I've seen this clue in the Evening Standard.

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