Loose lips letting slip quiet part (7)

I believe the answer is:

lissome

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'loose lips letting slip' is the definition.
I can't tell whether this definition defines the answer.

'quiet part' is the wordplay.
'quiet' becomes 'lis' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'part' becomes 'some' (I've seen this before).
'lis'+'some'='LISSOME'

Can you help me to learn more?

(Other definitions for lissome that I've seen before include "Lithe; nimble" , "flexible" , "Slim, supple, graceful" , "Oil mess (anag)" , "pliable" .)

I've seen this clue in The Independent.

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