Be sorry for one who is late (5)

I believe the answer is:

mourn

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'be sorry' is the definition.
The definition and answer can be both to do with human feelings as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?

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

Can you help me to learn more?

(Other definitions for mourn that I've seen before include "'Miss'" , "Show sorrow for loss" , "Lament loss" , "Keen" , "Express sadness" .)

I've seen this clue in The Globe and Mail.

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