One murdering masculine monarch (7)
I believe the answer is:
butcher
'one murdering' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'masculine monarch' is the wordplay.
'masculine' becomes 'butch' (similar in meaning).
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'butch'+'er'='BUTCHER'
Can you help me to learn more?
(Other definitions for butcher that I've seen before include "Killer" , "One sells meat" , "He cuts up and sells meat" , "Kill brutally" , "more manly" .)