Crossword Genius

ER abandons broken ventilator - that's incidental (3,5)

Ross

I believe the answer is:

not vital

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'incidental' is the definition.
Both the answer and definition are adjectives.
Perhaps you can see a link between them that I don't see?

I cannot understand how the rest of the clue works.

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I've seen this clue in The Irish Times.
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