Supporter not actually backing you up (5)

I believe the answer is:

stool

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'up' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.

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

Can you help me to learn more?

(Other definitions for stool that I've seen before include "item of furniture here" , "place butt on one?" , "Armless seat" , "Motion" , "Small seat" .)

I've seen this clue in the Evening Standard.

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