He'll make enquiries if one is unusually late (7)
I believe the answer is:
coroner
'he'll make enquiries if one is unusually late' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
This is all the clue.
Can you help me to learn more?
(Other definitions for coroner that I've seen before include "Inquest official" , "Inquest holder" , "He sits on cases" , "He presides over an inquest (7)" , "President of inquests" .)