Crossword Genius

Slithery thing left a mark climbing tree (6)

Ross

I believe the answer is:

mallee

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'tree' is the definition.
(mallee is a kind of tree)

'slithery thing left a mark climbing' is the wordplay.
'slithery thing' becomes 'eel' (I can't justify this - if you can you should give a lot more credence to this answer).
'left' becomes 'l' (common abbreviation).
'mark' becomes 'm'.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'eel'+'l'+'a'+'m'='eellam'
'eellam' written backwards gives 'MALLEE'.

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I've seen this clue in The Times.
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