Principles of mediocre editor (5)
I believe the answer is:
creed
'principles' is the definition.
(thesaurus)
'of mediocre editor' is the wordplay.
'of' indicates a hidden word.
'CREED' can be found hidden inside 'mediocre editor'.
(Other definitions for creed that I've seen before include "Statement of belief" , "System of religious belief" , "teleprinter" , "System of religious principles" , "Statement of beliefs" .)