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Child's pet is unreliable, but grand for second daughter (5)

Ross

I believe the answer is:

doggy

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'child's pet' is the definition.
(I've seen this in another clue)

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for doggy that I've seen is " Like canine animals".)

I've seen this clue in The Times.
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