Showed preference as to whom to give a seat (5)

I believe the answer is:

voted

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'showed' is the definition.
Although both the answer and definition are past participle verbs, I don't see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for voted that I've seen before include "Expressed a choice, political say" , "Took part in a ballot" , "Suggested" , "Expressed preference in a ballot" , "Made choice in ballot" .)

I've seen this clue in the Evening Standard.

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