Not the brightest — studies English! (5)
I believe the answer is:
dense
'not the brightest' is the definition.
(dense can mean stupid)
'studies english' is the wordplay.
'studies' becomes 'dens' (I've seen this in other clues).
'english' becomes 'E' (abbreviation).
'dens'+'e'='DENSE'
(Other definitions for dense that I've seen before include "Not very bright" , "Obtuse - impenetrable" , "Compressed - obtuse" , "Thick, compact or crowded" , "Thick; stupid" .)