Willow family tree (6)
I believe the answer is:
poplar
'willow family tree' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for poplar that I've seen before include "Slender tree with quivering leaves" , "tree found mostly in the northern hemisphere" , "part of London" , "Tree - London district" , "Tall fast-growing tree; East London area" .)