Study time causes depression (4)
I believe the answer is:
dent
'depression' is the definition.
('dent' can be a synonym of 'depression')
'study time' is the wordplay.
'study' becomes 'den' (I've seen this before).
'time' becomes 't'.
'den'+'t'='DENT'
'causes' acts as a link.
(Other definitions for dent that I've seen before include "A cavity" , "Impress" , "Injury to pride" , "Tend (anag.)" , "Impression left by impact" .)