Modernising, cheerful, sort of agency (8)

I believe the answer is:

updating

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'modernising' is the definition.
(I've seen this before)

'cheerful sort of agency' is the wordplay.
'cheerful' becomes 'up' (I've seen this in other clues).
'sort of agency' becomes 'dating' (I can't explain this - if you can you should believe this answer much more).
'up'+'dating'='UPDATING'

Can you help me to learn more?

(Other definitions for updating that I've seen before include "Giving latest info" , "Modernising" , "Reporting" .)

I've seen this clue in The Times.

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