Once about a fifth of a pound, it's actually less (5)
I believe the answer is:
ounce
'actually less' is the definition.
The answer and definition can be both quantities as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'once about a fifth of a pound' is the wordplay.
'about' indicates putting letters inside.
'a fifth of a pound' becomes 'u' (I can't justify this - if you can you should give a lot more credence to this answer).
'once' going around 'u' is 'OUNCE'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for ounce that I've seen before include "On cue (anag.)" , "weight? Surely not" , "Oz" , "Carnivore" , "Unit of weight - snow leopard" .)