Horse, possibly, nibbling root of willow tree (5)

I believe the answer is:

rowan

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'tree' is the definition.
(type of tree)

I can't explain the remainder of the clue.

(Other definitions for rowan that I've seen before include "-- Williams, theologian" , "Atkinson or Williams" , "Tree - archbishop" , "Mountain ash (at Lambeth!)" , "Tree with orange-red berry-like fruits" .)

I've seen this clue in The Times.

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