Bachelor properly in cheerful mode (8)

I believe the answer is:

brightly

'in cheerful mode' is the definition.
(bright can mean cheerful or upbeat)

'bachelor properly' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'properly' becomes 'rightly' (in the proper or right way).
'b'+'rightly'='BRIGHTLY'

(Another definition for brightly that I've seen is " with intelligence".)

I've seen this clue in The Guardian.

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