Sixteenth of a pound (5)
I believe the answer is:
ounce
'sixteenth of a pound' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ounce that I've seen before include "Unit equal to about 28 grams" , "Small unit of weight" , "Weight; big cat" , "one of a number in the pound" , "Small quantity - weight" .)