Preparation of whites only used by newcomer in guestroom (8)

I believe the answer is:

meringue

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'preparation of whites only' is the definition.
I can't judge whether this defines the answer.

'used by newcomer in guestroom' is the wordplay.
'used by' indicates the answer is hidden within the clue.
'MERINGUE' is hidden within 'newcomer in guestroom'.

Can you help me to learn more?

(Other definitions for meringue that I've seen before include "Confection based on egg whites" , "Sugary cake" , "Confection of beaten egg white and sugar" , "Basis of pavlova" , "Crisp baked mixture of beaten egg whites and sugar" .)

I've seen this clue in The Times.

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