Doctor has to run a car? (5)
I believe the answer is:
motor
'a car?' is the definition.
(motor can informally mean a car)
'doctor has to run' is the wordplay.
I cannot really see how this works, but
'doctor' could be 'mo' (medical officer) and 'mo' is located in the answer.
'to' is present in the answer.
'run' could be 'r' (cricket abbreviation) and 'r' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
(Other definitions for motor that I've seen before include "go quickly" , "Move fast" , "Car; travel quickly" , "Driver" , "Drive" .)