'60s youths adopting upper-class manner (5)

I believe the answer is:

modus

'manner' is the definition.
(thesaurus)

''60s youths adopting upper-class' is the wordplay.
''60s youths' becomes 'Mods' (members of 1960s subculture).
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'upper-class' becomes 'u' (similar in meaning).
'mods' enclosing 'u' is 'MODUS'.

(Another definition for modus that I've seen is " -- Operandi".)

I've seen this clue in The Times.

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