Sick person not having legal force (7)

I believe the answer is:

invalid

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'sick person' is the definition.
(I know that invalid is a type of sick person)

'not having legal force' is the wordplay.
'not' becomes 'in' (I can't explain this - if you can you should believe this answer much more).
'having legal force' becomes 'valid' (I've seen this in another clue).
'in'+'valid'='INVALID'

Can you help me to learn more?

(Other definitions for invalid that I've seen before include "Unsound; ill person" , "Worthless - sick person" , "Lacking legal force" , "Legally void" , "Disable" .)

I've seen this clue in The Irish News.

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