Has damaged Rowan (3)
I believe the answer is:
ash
'rowan' is the definition.
(I've seen this in another clue)
'has damaged' is the wordplay.
'damaged' is an anagram indicator.
'has' anagrammed gives 'ASH'.
(Other definitions for ash that I've seen before include "Grey-barked tree" , "Tree in danger" , "Tree with pale wood" , "Volcanic dust" , "Tree; fire residue" .)