Weighty form of nonet (5)

I believe the answer is:

tonne

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'weighty form' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

'of nonet' is the wordplay.
'of' indicates an anagram (the answer comes from or \'of\' the letters).
'nonet' with letters rearranged gives 'TONNE'.

Can you help me to learn more?

(Other definitions for tonne that I've seen before include "1000 kilograms" , "1,000 kg" , "Heavyweight" , "One thousand kilograms" , "A thousand kilogrammes" .)

I've seen this clue in the Evening Standard.

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