Refuse to accept position out of stupidity (7)
I believe the answer is:
density
'stupidity' is the definition.
(thesaurus)
'refuse to accept position' is the wordplay.
'refuse' becomes 'deny' ('deny' can be a synonym of 'refuse'**).
'to accept' indicates putting letters inside.
'position' becomes 'sit' (sitting is a kind of positioning).
'deny' enclosing 'sit' is 'DENSITY'.
'out of' acts as a link.
(Other definitions for density that I've seen before include "Number of items in a given area" , "It's usually measured in grammes per cc" , "Bulk" , "can be relative" , "Compactness - stupidity" .)