Philanderer's loveless regret (3)

I believe the answer is:

rue

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'regret' is the definition.
(synonyms)

I cannot understand how the rest of the clue works.

(Other definitions for rue that I've seen before include "Nice way" , "show you're sorry!" , "Lament; a herb" , "Fish eggs" , "Bitter-tasting herb" .)

I've seen this clue in The Times.

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