Long for a tree (4)
I believe the answer is:
pine
I believe this clue is a double definition.
'long for' is the first definition.
(thesaurus)
'a tree' is the second definition.
(pine is a kind of tree)
(Other definitions for pine that I've seen before include "Yearn for evergreen tree" , "A tree to grieve for" , "Tree, waste away" , "Yen - tree" , "Long - timber" .)