He once drove a medic back into jail (6)
I believe the answer is:
cabman
'he once drove' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a medic back into jail' is the wordplay.
'medic' becomes 'mb' (Bachelor of Medicine).
'back' shows that the letters should be reversed in order.
'into' means one lot of letters goes inside another.
'jail' becomes 'can' (the can is American slang for prison).
'a'+'mb'='amb'
'amb' back-to-front is 'bma'.
'bma' placed inside 'can' is 'CABMAN'.
Can you help me to learn more?
(Another definition for cabman that I've seen is " Driver in Hackney".)