One who helps Satanists out (9)
I believe the answer is:
assistant
'one who helps' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'satanists out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'satanists' with letters rearranged gives 'ASSISTANT'.
Can you help me to learn more?
(Other definitions for assistant that I've seen before include "second" , "one helps out" , "Deputy, subordinate" , "help required?" , "Attendant, helper" .)