Familiar way of working in study (5)
I believe the answer is:
demon
'familiar' is the definition.
(I've seen this before)
'way of working in study' is the wordplay.
'way of working' becomes 'mo' (abbreviation for modus operandi).
'in' means one lot of letters goes inside another.
'study' becomes 'den' (den can mean an office or study).
'mo' put inside 'den' is 'DEMON'.
(Other definitions for demon that I've seen before include "See 3" , "Familiar" , "Ghoul" , "Devil - master performer" , "Evil spirit, devil" .)