Study with pen restrains one's stupidity (7)
I believe the answer is:
density
'stupidity' is the definition.
(thesaurus)
'study with pen restrains one's' is the wordplay.
'study' becomes 'den' (I've seen this before).
'with' says to put letters next to each other.
'pen' becomes 'sty' (sty is a kind of pen**).
'restrains' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'sty' going around 'i' is 'sity'.
'den'+'sity'='DENSITY'
(Other definitions for density that I've seen before include "Bulk" , "Number of items in a given area" , "The compactness of a substance" , "The quantity of a given area" , "showing obtuseness" .)