Most cheerful relation, one taken in by joke (9)

I believe the answer is:

jauntiest

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'most cheerful' is the definition.
('jaunty' can be similar in meaning to 'cheerful')

'relation one taken in by joke' is the wordplay.
'relation' becomes 'aunt' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'taken in' indicates putting letters inside.
'by joke' becomes 'jest' ('jest' can be a synonym of 'joke'. I am not sure about the 'by' bit.**).
'aunt'+'i'='aunti'
'aunti' placed within 'jest' is 'JAUNTIEST'.

Can you help me to learn more?

(Other definitions for jauntiest that I've seen before include "most cheerful" , "most lively" .)

I've seen this clue in The Times.

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