Crossword Genius

Was Jim Morrison one getting closer? (4)

Ross

I believe the answer is:

door

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'one getting closer?' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for door that I've seen before include "Jim Morrison?" , "Threshold" , "Alternative to knocker" , "Barrier to open and close" , "Entrance to room, house etc" .)

I've seen this clue in The Sun.
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