Postal scale units (6)
I believe the answer is:
ounces
'postal scale units' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ounces that I've seen before include "Twelfths of a pound" , "Imperial measures of weight" , "Small weights in UNESCO" , "Light weights or very small amounts" , "Small units of weight" .)