How the watery sunlight will come through the window? (6)

I believe the answer is:

stream

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'window?' is the definition.
Although both the answer and definition are singular nouns, I cannot 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 stream that I've seen before include "class division" , "Fly" , "Flow of water on land" , "group of pupils split by ability" , "Small watercourse" .)

I've seen this clue in the Evening Standard.

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