Employer requiring superior, faultless service (4)
I believe the answer is:
user
'employer' is the definition.
(someone who employs or uses something)
'superior faultless service' is the wordplay.
'superior' becomes 'U' (U can mean upper-class, as in U and non-U speech).
'faultless service' becomes 'ser' (I can't justify this - if you can you should give a lot more credence to this answer).
'u'+'ser'='USER'
'requiring' is the link.
Can you help me to learn more?
(Other definitions for user that I've seen before include "One exploiting others" , "Drug taker" , "One who works an appliance" , "Utiliser" , "Tech support caller?" .)