Evil spirit seen for a short time in the study (5)
I believe the answer is:
demon
'evil spirit' is the definition.
(I know that demon are evil)
'short time in the study' is the wordplay.
'short time' becomes 'mo' (as in 'just a mo').
'in the' is an insertion indicator.
'study' becomes 'den' (term for a study or office).
'mo' placed into 'den' is 'DEMON'.
'seen for a' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "malignant spirit" , "Monster" , "Evil spirit or cruel person" , "Imp" , "Evildoer" .)