In study, this fiend has his own way of working (5)
I believe the answer is:
demon
'in study this fiend has his own way of working' is the definition.
'demon' can be an answer for 'fiend' (synonyms). I'm not certain of the remainder of the definition.
'in study this fiend has' is the wordplay.
'in' is an insertion indicator.
'study' becomes 'den' (term for a study or office).
'this fiend has' becomes 'mo' (I can't justify this - if you can you should believe this answer much more).
'den' enclosing 'mo' is 'DEMON'.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "Fallen angel" , "Familiar" , "Genius" , "Fiend, devil" , "Wizard" .)