Imperial weight (5)
I believe the answer is:
ounce
'imperial weight' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ounce that I've seen before include "16 for a pound" , "Light weight coming in on cue" , "Fraction of a pound" , "On cue (anag.)" , "Twelfth of a pound" .)