Crossword Genius

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

Ross

I believe the answer is:

stream

'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", "'Current, flow (6)'", "group of schoolchildren", "schoolchildren grouped together", "Flow from master".)

I've seen this clue in Evening Standard.
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