She lives with her mother, that is (6)

I believe the answer is:

maisie

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'she lives with her mother that is' is the definition.
'Maisie' can be an answer for 'she'. I'm not certain of the remainder of the definition.

'with her mother that is' is the wordplay.
'with' indicates putting letters inside.
'her' becomes 'si' (I can't explain this - if you can you should believe this answer much more).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'ma'+'ie'='maie'
'si' going inside 'maie' is 'MAISIE'.

Can you help me to learn more?

(Another definition for maisie that I've seen is " James's girl".)

I've seen this clue in the Evening Standard.

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