Maybe GI initially dropping bit of litter (4)

I believe the answer is:

runt

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'litter' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for runt that I've seen before include "Weakling of the litter" , "Smallest squealer" , "Smallest animal in a litter" , "Weakest animal in a litter" , "Weak little squealer" .)

I've seen this clue in The Independent.

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