Doubtless, mistake offends client (4-9)
I believe the answer is:
self-confident
'doubtless' is the definition.
(having few doubts about one's abilities)
'mistake offends client' is the wordplay.
'mistake' indicates anagramming the letters.
'offends'+'client'='offendsclient'
'offendsclient' anagrammed gives 'SELF-CONFIDENT'.
(Other definitions for self-confident that I've seen before include "Having faith in one's own abilities" , "Sure of one's abilities" , "Sure of oneself" , "Poised" , "assured" .)