Timely sound, maybe followed by a squeak (3)

I believe the answer is:

pip

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'squeak' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for pip that I've seen before include "Great Expectations hero" , "Dickensian hero" , "Narrowly defeat" , "Palindromic pear seed" , "Greenwich time signal" .)

I've seen this clue in the Evening Standard.

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