One is shown it dismissively (4)
I believe the answer is:
door
'one' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for door that I've seen before include "Portal" , "Entry point" , "Opener" , "Threshold" , "Means of exit and entrance" .)