Billy's favourite nurse (5)

I believe the answer is:

nanny

I'm a little stuck... Click here to teach me more about this clue!

This clue could be a double definition.

'billy's' is the first definition.
(thesaurus)

'favourite nurse' is the second definition.
'nanny' can be an answer for 'nurse' (synonyms). I am unsure of the 'favourite' bit.

Can you help me to learn more?

(Other definitions for nanny that I've seen before include "cares for child?" , "Posh child minder or female goat" , "Carer for children" , "Childminder - goat" , "Person minding another's child" .)

I've seen this clue in the Evening Standard.

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