Kind of ton (6)
I believe the answer is:
metric
'kind of ton' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for metric that I've seen before include "System using litre and gram as units" , "Consisting of verses; pertaining to one system of measurement" , "System of weights and measures" , "Non-Imperial system of measurement" , "Like the SI system" .)