Squat plastered in refuse — how stupid? (7)
I believe the answer is:
density
'how stupid?' is the definition.
(dense can mean stupid)
'squat plastered in refuse' is the wordplay.
'squat' becomes 'sit' (I've seen this in other clues).
'plastered in' indicates putting letters inside (plaster can mean to cover or coat).
'refuse' becomes 'deny' (synonyms).
'sit' inserted within 'deny' is 'DENSITY'.
(Other definitions for density that I've seen before include "The compactness of a substance" , "there's stupidity for you!" , "Bulk" , "Mass per volume" , "can be relative" .)