Twentieth of a ton (13)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
hundredweight
(Other definitions for hundredweight that I've seen before include "Just over fifty kilograms" , "British unit of 112 pounds" , "112 pounds (cwt)" , "Number" , "cwt (there are 20 of them to the Imperial and American ton)" .)