Studies wrong set, being most stupid (7)
I believe the answer is:
densest
'most stupid' is the definition.
('dense' can be similar in meaning to 'stupid')
'studies wrong set' is the wordplay.
'studies' becomes 'dens' (I've seen this before).
'wrong' indicates anagramming the letters.
'set' anagrammed gives 'est'.
'dens'+'est'='DENSEST'
'being' is the link.
(Other definitions for densest that I've seen before include "Most closely packed" , "extremely thick" .)