Regret sounding bit stressed out? (9)

I believe the answer is:

penitence

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'regret' is the definition.
(penitence is a kind of regret)

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

(Other definitions for penitence that I've seen before include "shame" , "Contriteness" , "Contrition, remorse" , "A show of regret for wrongdoing" , "display of remorse" .)

I've seen this clue in The Telegraph.

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