By hollow tree, a mystery pair left tissue in forest (5)

I believe the answer is:

xylem

I'm a little stuck... Click here to teach me more about this clue!

'tissue in forest' is the definition.
'xylem' can be an answer for 'tissue' (xylem is a kind of tissue). I'm not sure about the 'in forest' bit.

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for xylem that I've seen before include "trees - it's natural for them" , "Woody plant tissue" , "wood" , "Woody part of plants" .)

I've seen this clue in the Sydney Morning Herald.

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