When there's sun, habitually will thaw (6)

I believe the answer is:

soften

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'thaw' is the definition.
(thesaurus)

I don't understand the rest of the clue.

(Other definitions for soften that I've seen before include "Tenderise" , "Become gentler" , "Tone down" , "Mollify" , "To moderate" .)

I've seen this clue in the Evening Standard.

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