For doubters, head of audit worked out costings (9)
I believe the answer is:
agnostics
'for doubters' is the definition.
('agnostic' can be a synonym of 'doubter')
'head of audit worked out costings' is the wordplay.
'head of' indicates taking the first letters.
'worked out' indicates an anagram.
The first letter of 'audit' is 'a'.
'costings' is an anagram of 'gnostics'.
'a'+'gnostics'='AGNOSTICS'
(Other definitions for agnostics that I've seen before include "They claim one can't have true knowledge of the existence of God" , "They refuse to believe" , "Sceptics" , "They don't know if God exists or not" .)