One-fifth of a ton (6)
I believe the answer is:
twenty
This clue could be a double definition.
'one-fifth' is the first definition.
The answer and definition can be both quantities as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'a ton' is the second definition.
The definition and answer can be both quantities as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'of' acts as a link.
Can you help me to learn more?
(Other definitions for twenty that I've seen before include "made by Q on a light blue square" , "Hundredweights to a ton" , "Dozen plus eight" , "No longer a teenager" , "previous clue?" .)