Shorten study, stupid (8)
I believe the answer is:
condense
'shorten' is the definition.
(condensing is a kind of shortening)
'study stupid' is the wordplay.
'study' becomes 'con' (I've seen this before).
'stupid' becomes 'dense' ('dense' can be similar in meaning to 'stupid').
'con'+'dense'='CONDENSE'
(Other definitions for condense that I've seen before include "Concentrate" , "Summarise" , "Change from vapour to liquid" , "Make more concentrated, or express in fewer words" , "Abridge" .)