Crossword Genius

Invalid's physician finally taking over from hospital in northern city (4)

Ross

I believe the answer is:

null

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'invalid's' is the definition.
(similar in meaning)

I cannot understand how the remainder of the clue works.

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(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" .)

I've seen this clue in The Independent.
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