Employer requiring superior and faultless service (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'superior and faultless service' is the wordplay.
'superior' becomes 'U' (U can mean upper-class, as in U and non-U speech).
'and' means one lot of letters go next to another.
'faultless service' becomes 'ser' (I can't explain this - if you can you should believe this answer much more).
'u'+'ser'='USER'
'requiring' acts as a link.
Can you help me to learn more?
(Other definitions for user that I've seen before include "one has to employ" , "Utiliser" , "One operating something" , "One who exploits others" , "With 1a [NEW] he'll need different login" .)