One employing American English regularly at first (4)
I believe the answer is:
user
'one employing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'american english regularly at first' is the wordplay.
'american' becomes 'us' (United States).
'at first' says to take the initial letters.
The initial letters of 'english regularly' is 'er'.
'us'+'er'='USER'
Can you help me to learn more?
(Other definitions for user that I've seen before include "One operating something" , "Manipulator" , "One who employs" , "Person who takes drugs" , "Drug addict (colloq.)" .)