Thoroughly stilted but polite (4-8)

I believe the answer is:

well-mannered

'polite' is the definition.
('well-mannered' can be similar in meaning to 'polite')

'thoroughly stilted' is the wordplay.
'thoroughly' becomes 'well' (I've seen this before).
'stilted' becomes 'mannered'.
'well'+'mannered'='WELL-MANNERED'

'but' is the link.

(Another definition for well-mannered that I've seen is " Polite".)

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