Weight system (11)
I believe the answer is:
avoirdupois
'weight system' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for avoirdupois that I've seen before include "Imperial system of weights - unwanted fat" , "Pounds/ounces system of weights" , "sort of weight" , "Non-metric system of weights" .)