Still, a reticent person needs a change of heart (4)

I believe the answer is:

calm

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'still' is the definition.
(synonyms)

I can't explain the remainder of the clue.

(Other definitions for calm that I've seen before include "Peaceful and unexcited" , "Still and quiet" , "Cool, not excited" , "'After a storm comes a ....'" , "Pacific" .)

I've seen this clue in The Guardian.

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