One who exploits you, me and Her Majesty (4)
I believe the answer is:
user
'one who exploits you' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'me and her majesty' is the wordplay.
'me' becomes 'us' ('us' can mean 'me' in some dialects).
'and' is a charade indicator (letters next to each other).
'her majesty' becomes 'ER' (Elizabeth Regina).
'us'+'er'='USER'
Can you help me to learn more?
(Other definitions for user that I've seen before include "Right long-established" , "Patron" , "Exploiter (of others)" , "junkie" , "Druggie" .)