Serious enough to make one late (5)

I believe the answer is:

fatal

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'serious' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

(Other definitions for fatal that I've seen before include "Unavoidable" , "making one late" , "critical" , "Pernicious" , "Resulting in disaster or death" .)

I've seen this clue in the Evening Standard.

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