One takes it to heart (6)

I believe the answer is:

united

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'one' is the definition.
(synonyms)

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

(Other definitions for united that I've seen before include "See 1" , "Various football teams" , "Allied or combined" , "Of like mind" , "collective" .)

I've seen this clue in the Evening Standard.

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