One doubts a costing is wrong (8)
I believe the answer is:
agnostic
'one doubts' is the definition.
(I've seen this before)
'a costing is wrong' is the wordplay.
'is wrong' indicates an anagram.
'costing' is an anagram of 'gnostic'.
'a'+'gnostic'='AGNOSTIC'
(Other definitions for agnostic that I've seen before include "He finds it difficult to believe" , "Unbeliever; sceptic" , "Song I act (anag)" , "Coasting away with non-believer" , "Religious sceptic" .)