In the end, perhaps, the modus operandi seems fiendish! (5)
I believe the answer is:
demon
'seems fiendish' is the definition.
I can't judge whether this definition defines the answer.
'in the end perhaps the modus operandi' is the wordplay.
'in' indicates putting letters inside.
'perhaps' is an anagram indicator.
'the modus operandi' becomes 'mo' (common abbreviation).
'end' with letters rearranged gives 'den'.
'den' placed around 'mo' is 'DEMON'.
Can you help me to learn more?
(Other definitions for demon that I've seen before include "beast" , "malignant spirit" , "Skilful person" , "Fanatic; devil" , "Fiend, devil" .)