Floating rubbish? (4)

I believe the answer is:

junk

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'rubbish?' is the definition.
(I know that junk 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 junk that I've seen before include "Schlock" , "Clutter - Chinese boat" , "ship" , "Accumulated rubbish" , "Sailing vessel; rubbish" .)

I've seen this clue in the Evening Standard.

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