Crossword Genius

It grows in midstream, we’re told (4)

Ross

I believe the answer is:

tree

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'it grows' is the definition.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tree that I've seen before include "Genealogist's work" , "eg Lime" , "Plane perhaps" , "Large plant with trunk" , "trod the boards" .)

I've seen this clue in the Evening Standard.
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