For paper promises, faultless service is no joke! (7)
I believe the answer is:
serious
'is no joke' is the definition.
(thesaurus)
'for paper promises' is the wordplay.
'for' becomes 'ser' (I can't justify this - if you can you should give a lot more credence to this answer).
'paper promises' becomes 'ious' (I've seen this in another clue).
'ser'+'ious'='SERIOUS'
'faultless service' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for serious that I've seen before include "Not light or frivolous" , "Important -- grave" , "Solemn, thoughtful" , "Earnest - important" , "Earnest; vital" .)