Has turned into tree (3)
I believe the answer is:
ash
'tree' is the definition.
(ash is a kind of tree)
'has turned' is the wordplay.
'turned' indicates an anagram.
'has' with letters rearranged gives 'ASH'.
'into' acts as a link.
(Other definitions for ash that I've seen before include "Tree; cinders" , "Grey colour" , "Volcanic dust" , "What's left of the fire" , "Silvery grey" .)