Nonet in a heavy arrangement (5)

I believe the answer is:

tonne

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'a heavy arrangement' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'nonet' is the wordplay.
'nonet' anagrams to 'TONNE'.
But, I am not sure how this is indicated.

'in' acts as a link.

Can you help me to learn more?

(Other definitions for tonne that I've seen before include "1000 kilograms" , "Great weight" , "Mass" , "Metric weight unit" , "Heavyweight" .)

I've seen this clue in the Evening Standard.

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