Evergreen tree harbouring strain from the east (6)

I believe the answer is:

myrtle

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

I cannot understand how the rest of the clue works.

(Other definitions for myrtle that I've seen before include "Shrub with white flowers and aromatic berries" , "Girl" , "Termly (anag.)" , "Shiny evergreen shrub" , "Try Mel for the flowering shrub" .)

I've seen this clue in The Independent.

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