Invalid's physician finally taking over from hospital in northern city (4)
I believe the answer is:
null
'invalid's' is the definition.
(similar in meaning)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for null that I've seen before include "Not valid" , "Lacking any legally binding force" , "Having the value zero -- void" , "Immaterial and void" , "Invalid, void" .)