Obstruction that may be knocked off? (5)
I believe the answer is:
block
'obstruction' is the definition.
(block is a kind of obstruction)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for block that I've seen before include "building development" , "Cube" , "City area; prevent flow" , "Obstruct" , "Jam" .)