Freight weight unit (3)
I believe the answer is:
ton
'freight weight unit' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ton that I've seen before include "A slangy 100 mph - heavy" , "Style" , "A slang 100 mph" , "Heavy weight or 100mph" , "A slang 100mph - heavy" .)