Being chronically sick is having no effect (7)

I believe the answer is:

invalid

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'having no effect' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for invalid that I've seen before include "Person disabled by injury or sickness" , "Worthless - sick person" , "Not telling" , "Nugatory" , "case? Doesn't sound like it" .)

I've seen this clue in The Times.

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