Trees lose weight, that's true (3)

I believe the answer is:

yes

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'true' 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 rest of the clue.

Can you help me to learn more?

(Other definitions for yes that I've seen before include "Indeed" , "Of course" , "I agree" , "Affirmative reply" , "That's 27 [RIGHT]" .)

I've seen this clue in The Times.

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