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Piece of wood: heavy weight that floats in the sea (8)

Ross

I believe the answer is:

plankton

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'floats in the sea' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.

'piece of wood heavy weight' is the wordplay.
'piece of wood' becomes 'plank'.
'heavy weight' becomes 'ton' (a ton weighs a lot).
'plank'+'ton'='PLANKTON'

'that' acts as a link.

Can you help me to learn more?

(Other definitions for plankton that I've seen before include "Microscopic sea food" , "Microscopic organisms in seawater" , "Food source for aquatic creatures" , "Microscopic organic life that floats in sea" , "Tiny creatures" .)

I've seen this clue in The Guardian.
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