Person who employs American head of state (4)
I believe the answer is:
user
'person who employs' is the definition.
'user' can be an answer for 'person' (user is a kind of person). I am not certain of the rest of the definition.
'american head of state' is the wordplay.
'american' becomes 'us' (abbreviation for United States).
'head of state' becomes 'er' (abbreviation for Elizabeth Regina).
'us'+'er'='USER'
Can you help me to learn more?
(Other definitions for user that I've seen before include "Drug addict (colloq.)" , "Person taking drugs" , "Manipulator" , "One who works an appliance" , "Person employing something" .)