Unit of 1,000 kilograms (5)

I think it could be either:

Which length description are you looking for?:

6,3

tonne

'unit of 1000 kilograms' is the definition.
(I've seen this in another clue)

This is all the clue.

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

I've seen this clue in The Times.

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