Regret being harsh, expelling daughter (3)

I believe the answer is:

rue

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'regret' is the definition.
(to rue something is to regret it)

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

(Other definitions for rue that I've seen before include "Feel regret for the herb" , "Regret or repent" , "Feel remorse" , "Nice way" , "The 'herb of grace'" .)

I've seen this clue in The Independent.

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