Studies, mostly stupid (4)
I believe the answer is:
dens
'studies' is the definition.
(I've seen this in another clue)
'mostly stupid' is the wordplay.
'mostly' means to remove the last letter (most but not all of the word).
'stupid' becomes 'dense' (similar in meaning).
'dense' with its last letter taken off is 'DENS'.
(Other definitions for dens that I've seen before include "Habitats of some wild animals" , "Man's" , "Lairs" , "Studies" , "Habitations of wild animals" .)