They’re young? Rubbish! (6)

I believe the answer is:

litter

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'rubbish' is the definition.
(I know that litter is a type of rubbish)

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

Can you help me to learn more?

(Other definitions for litter that I've seen before include "Rubbish strewn about the streets" , "Do the kittens create rubbish?" , "Carelessly-dropped rubbish" , "Trash left lying about" , "Odds and ends of rubbish left lying about" .)

I've seen this clue in the Evening Standard.

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