Tall, slender tree (6)
I believe the answer is:
poplar
'tall slender tree' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for poplar that I've seen before include "Tall fast-growing tree; East London area" , "Deciduous tree" , "part of London" , "The tree is almost liked" , "Tree as tulip-tree, aspen" .)