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 defines the answer.
'in the end perhaps the modus operandi' is the wordplay.
'in' means one lot of letters goes inside another.
'perhaps' indicates an anagram.
'the modus operandi' becomes 'MO' (common abbreviation).
'end' is an anagram of '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" .)