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Tree
expert’s
concoction
of
citrus
and
oil-tar
rub
(15)
Tree expert’s concoction of citrus and oil-tar rub (15)
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The definition is at the beginning of the clue.
How the clue works
Right, so Tree expert is your definition there, pointing to an arboriculturist. Now "concoction" is the anagram flag, telling you to chuck together all the letters from "citrus," "oil," "tar" and "rub" and give them a good shuffle — do that and they fall into ARBORICULTURIST.