Study to comprehend modus operandi in evil spirit (5)
I believe the answer is:
demon
'evil spirit' is the definition.
(I know that demon is a type of evil spirit)
'study to comprehend modus operandi' is the wordplay.
'study' becomes 'den' (term for a study or office).
'to comprehend' means one lot of letters goes inside another (comprehend can mean to contain or include).
'modus operandi' becomes 'MO' (common abbreviation).
'den' placed around 'mo' is 'DEMON'.
'in' acts as a link.
(Other definitions for demon that I've seen before include "See 3" , "beast" , "Evil spirit or cruel person" , "malignant spirit" , "Evildoer" .)