One checking car on hill (7)
I believe the answer is:
auditor
'one checking' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'car on hill' is the wordplay.
'car' becomes 'audi' (Audi is a German car brand).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'hill' becomes 'tor' (tor is a kind of hill).
'audi'+'tor'='AUDITOR'
Can you help me to learn more?
(Other definitions for auditor that I've seen before include "Worker in accounts" , "Finance officer" , "He'll check the accounts" , "I'll check expense involved" , "Listener; scrutineer" .)