Pound portion (5)
I believe the answer is:
ounce
'pound portion' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ounce that I've seen before include "One-sixteenth of pound" , "Snow-leopard" , "One sixteenth of a pound" , "one of a number in the pound" , "Fraction of a pound" .)