The principle of no credit (8)
I believe the answer is:
doctrine
'the principle' is the definition.
(thesaurus)
'of no credit' is the wordplay.
'of' indicates an anagram (the answer comes from or 'of' the letters).
'no'+'credit'='nocredit'
'nocredit' is an anagram of 'DOCTRINE'.
(Other definitions for doctrine that I've seen before include "System of beliefs" , "No credit given to belief" , "Body of religious teaching" , "Article of faith" , "Set of religious principles" .)