Approved of popular quartet taking a fifth (2,6)

I believe the answer is:

in favour

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

'popular quartet taking a fifth' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'quartet' becomes 'four' (synonyms).
'taking' is an insertion indicator.
'a fifth' becomes 'av' (I can't explain this - if you can you should give a lot more credence to this answer).
'four' enclosing 'av' is 'favour'.
'in'+'favour'='IN FAVOUR'

Can you help me to learn more?

(Other definitions for in favour that I've seen before include "Pro" , "Popular" , "What persona grata would be" , "Highly regarded" , "Approved of" .)

I've seen this clue in the Sydney Morning Herald.

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