Unit of weight around a ton, about right? (5)

I believe the answer is:

carat

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

'around a ton about right?' is the wordplay.
'around' is a reversal indicator.
'ton' becomes 'c'.
'about right?' becomes 'rat' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'c'='ac'
'ac' reversed gives 'ca'.
'ca'+'rat'='CARAT'

Can you help me to learn more?

(Other definitions for carat that I've seen before include "Unit of weight used for gems" , "Not a large weight" , "Diamond weight unit" , "Gem weight unit" , "Unit of weight of gemstones" .)

I've seen this clue in The Telegraph.

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