German car with corrosion knocked back inspector (7)
I believe the answer is:
auditor
'inspector' is the definition.
(thesaurus)
'german car with corrosion knocked back' is the wordplay.
'german car' becomes 'audi' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'corrosion' becomes 'rot' (both can mean decay).
'knocked back' says the letters should be written backwards.
'rot' reversed gives 'tor'.
'audi'+'tor'='AUDITOR'
(Other definitions for auditor that I've seen before include "One who hears can check the books" , "He'll check the accounts" , "One qualified to check accounts" , "Account inspector" , "Business accountant" .)