Harass evil and good monarch (6)
I believe the answer is:
badger
'harass' is the definition.
(I know that badger is a more specific form of the action bedevil)
'evil and good monarch' is the wordplay.
'evil' becomes 'bad' (associated in meaning).
'and' means one lot of letters go next to another.
'good' becomes 'g' (abbreviation).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'bad'+'g'+'er'='BADGER'
(Other definitions for badger that I've seen before include "Pester -- nocturnal mammal" , "Black and white animal called Brock" , "A mammal; harry" , "Burrowing carnivore" , "Black and white striped animal" .)