Flatten down (and put up again?) (5)
I believe the answer is:
level
'flatten' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for level that I've seen before include "Storey - stage" , "Horizontal line" , "Having a flat, horizontal surface" , "Smooth - sort of crossing" , "Equal; flat" .)