Smoothing round knot, not missing floorboards (8)
I believe the answer is:
planking
'floorboards' is the definition.
(I've seen this before)
'smoothing round knot not missing' is the wordplay.
'smoothing' becomes 'planing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'round' indicates putting letters inside.
'missing' is a deletion indicator.
'knot' with 'not' taken out is 'k'.
'planing' going around 'k' is 'PLANKING'.
Can you help me to learn more?
(Other definitions for planking that I've seen before include "Basic floor" , "Boards" , "Floorboards" , "lots of wood" .)