Refuse to accept position? That shows stupidity (7)
I believe the answer is:
density
'stupidity' is the definition.
(thesaurus)
'refuse to accept position?' is the wordplay.
'refuse' becomes 'deny' (synonyms).
'to accept' indicates putting letters inside.
'position?' becomes 'sit' (sitting is a kind of positioning).
'deny' going around 'sit' is 'DENSITY'.
'that shows' acts as a link.
(Other definitions for density that I've seen before include "'Thickness, concentration (7)'" , "It's usually measured in grammes per cc" , "The compactness of a substance" , "Degree of compactness" , "Mass per volume" .)