Devilish way of working in study (5)
I believe the answer is:
demon
'devilish way of working in study' is the definition.
I can't tell whether this defines the answer.
'way of working in study' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'in' indicates putting letters inside.
'study' becomes 'den' (term for a study or office).
'mo' going within 'den' is 'DEMON'.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "ace" , "Expert" , "Fiend, devil" , "Fallen angel" , "Wizard" .)