Unpunctual son ignored reprimand (4)

I believe the answer is:

late

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'unpunctual' is the definition.
(similar in meaning)

I cannot understand how the remainder of the clue works.

(Other definitions for late that I've seen before include "Dead - tardy" , "Dead or not punctual" , "Unpunctual or dead (4)" , "Far into the night" , "Passed on (but still not passed by!)" .)

I've seen this clue in The Hindu.

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