Troublemaker, one Boris Johnson? (3)

I believe the answer is:

imp

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'troublemaker' is the definition.
(little devil)

'one boris johnson?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'boris johnson?' becomes 'mp' (I can't explain this - if you can you should believe this answer much more).
'i'+'mp'='IMP'

Can you help me to learn more?

(Other definitions for imp that I've seen before include "Malicious spirit" , "Mischievous child" , "A small mischievous devil" , "Goblin" , "pickle" .)

I've seen this clue in The Telegraph.

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