American Queen's employer (4)
I believe the answer is:
'employer' is the definition.
(someone who employs or uses something)
'american queen's' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'queen' becomes 'ER' (abbreviation for Elizabeth Regina).
(Other definitions for user that I've seen before include "Operator; drug addict" , "One taking drugs" , "Drug addict (colloq.)" , "One exploiting others" , "Exploiter (of others)" .)