Supporter not actually backing you up (5)
I believe the answer is:
'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" .)