Room tiny arachnid enters backwards — it has four dimensions (5-4)

I believe the answer is:

space-time

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'arachnid enters backwards it has four dimensions' is the definition.
I can't judge whether this defines the answer.

'room tiny' is the wordplay.
'room' becomes 'space' (I've seen this before).
'tiny' becomes 'time' (I can't explain this - if you can you should give a lot more credence to this answer).
'space'+'time'='SPACE-TIME'

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