Obscene imperial measure (5)
I believe the answer is:
gross
This clue could be a double definition.
'obscene' is the first definition.
(thesaurus)
'imperial measure' is the second definition.
'gross' can be an answer for 'measure' (gross is a kind of measure). I am not certain of the 'imperial' bit.
Can you help me to learn more?
(Other definitions for gross that I've seen before include "Flagrant - unpleasant - 144" , "exclamation of disgust" , "Coarse; repulsive" , "square?" , "Description of crude" .)