In the end, perhaps, the modus operandi seems fiendish! (5)

I believe the answer is:

demon

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'seems fiendish' is the definition.
I can't tell whether this defines the answer.

'in the end perhaps the modus operandi' is the wordplay.
'in' is an insertion indicator.
'perhaps' indicates anagramming the letters.
'the modus operandi' becomes 'MO' (common abbreviation).
'end' with letters rearranged gives 'den'.
'den' going 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" .)

I've seen this clue in the Evening Standard.

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