Knowing that costing is wrong (7)
I believe the answer is:
gnostic
'knowing' is the definition.
(thesaurus)
'costing is wrong' is the wordplay.
'is wrong' indicates an anagram.
'costing' with letters rearranged gives 'GNOSTIC'.
'that' is the link.
(Other definitions for gnostic that I've seen before include "Occult" , "Having special mystical knowledge, costing a lot" , "Relating to mystical knowledge" .)