Offender getting faultless service out of a pub (6)
I believe the answer is:
sinner
'offender' is the definition.
(I know that sinner is a type of offender)
'faultless service out of a pub' is the wordplay.
'faultless service' becomes 'ser' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'out of' is an insertion indicator (some letters on the outside of others).
'a pub' becomes 'inn' (an inn is a type of pub).
'ser' going around 'inn' is 'SINNER'.
'getting' is the link.
Can you help me to learn more?
(Other definitions for sinner that I've seen before include "One who breaks moral law" , "Transgressor of the moral law" , "One in breach of moral law" , "One who breaches the moral law" , "Immoral transgressor" .)