From stupidity, don’t agree to sit in (7)
I believe the answer is:
density
'from stupidity don't agree' is the definition.
I can't tell whether this defines the answer.
'to sit in' is the wordplay.
'to' becomes 'deny' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'deny' enclosing 'sit' is 'DENSITY'.
Can you help me to learn more?
(Other definitions for density that I've seen before include "Number of items in a given area" , "Closeness" , "Concentration, compactness" , "there's stupidity for you!" , "Bulk" .)