One filling hollow first? (7)
I believe the answer is:
dentist
'one filling' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'hollow first?' is the wordplay.
'hollow' becomes 'dent' (both can mean an impression or indentation).
'first?' becomes 'ist' (ist resembles 1st).
'dent'+'ist'='DENTIST'
Can you help me to learn more?
(Other definitions for dentist that I've seen before include "Teeth specialist" , "Tooth surgeon" , "part of the medical team" , "Practitioner who looks down in the mouth?" , "Tooth doctor" .)