Crossword Genius

Chum's introduced to lady in pants showing vivacity (10)

Ross

I believe the answer is:

animatedly

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'chum's introduced to lady in pants showing vivacity' is the definition.
I can't judge whether this definition defines the answer.

'chum's introduced to lady in pants' is the wordplay.
'chum's' becomes 'mate' (I can't justify this - if you can you should give a lot more credence to this answer).
'introduced to' indicates putting letters inside.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
'lady'+'in'='ladyin'
'ladyin' anagrammed gives 'anidly'.
'mate' inserted within 'anidly' is 'ANIMATEDLY'.

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I've seen this clue in The Telegraph.
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