Suddenly realise what grows on tree (4)

I believe the answer is:

twig

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'suddenly realise' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for twig that I've seen before include "Informally grasp meaning" , "Get the idea" , "small piece in plane, say" , "Tiny end of branch" , "Small shoot or branch" .)

I've seen this clue in The Times.

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