Has turned into a tree (3)
I believe the answer is:
ash
'a tree' is the definition.
(ash is a kind of tree)
'has turned' is the wordplay.
'turned' is an anagram indicator.
'has' anagrammed gives 'ASH'.
'into' is the link.
(Other definitions for ash that I've seen before include "In fireplace it may be" , "Tree; cinders" , "remains after forest fire?" , "Hardwood" , "Burnt dust" .)