Imperial weights (6)
I believe the answer is:
ounces
'imperial weights' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ounces that I've seen before include "Sixteen to a pound" , "Small weights in UNESCO" , "There are 12 in a pound" , "More than one cat" , "Twelfths of a pound" .)