About to treat the insane foreigner (8)

I believe the answer is:

estonian

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'foreigner' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

'about to treat the insane' is the wordplay.
'about' indicates putting letters inside.
'treat' is an anagram indicator.
'insane' anagrammed gives 'esnian'.
'to' put within 'esnian' is 'ESTONIAN'.

Can you help me to learn more?

(Other definitions for estonian that I've seen before include "From Tallinn, for example" , "Balt" , "Eastern Europe" , "Native of a Baltic republic" , "E. European" .)

I've seen this clue in the Evening Standard.

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