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Tops of yew and fir's fibrous bark (found in the Highlands) (4)

Ross

I believe the answer is:

yaff

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'bark found in the highlands' is the definition.
I can't tell whether this definition defines the answer.

'tops of yew and fir's fibrous' is the wordplay.
'tops of' says to take the initial letters.
The first letters of 'yew and firs fibrous' is 'YAFF'.

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