Scots own a certain part (3)

I believe the answer is:

ain

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'scots own' is the definition.
I can't tell whether this defines the answer.

'a certain part' is the wordplay.
'certain part' becomes 'in' (synonyms. I am not sure about the 'certain' bit.).
'a'+'in'='AIN'

Can you help me to learn more?

(Other definitions for ain that I've seen before include "26 [ ]" , "Your own in Scottish dialect" , "The Scot's own" , "Own (Scot)" , "Glaswegian personal" .)

I've seen this clue in the Evening Standard.

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