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How do you understand "one a baby who..." in.. ? - baby pearl earrings

.. Sentence:
"For example, I had ten of us now in the house, a laundry room with a baby that was dirtier than the others."
(young girl with a Pearl Earring, Tracy Chevalier, p. 21)

I think it's about "a" meaning ", the" how "the only person who" needs, but can not find a corresponding definition in my dictionaries. Is this a way to talk to?

We thank you, if you can help me!