Dendrologist's study (5)
I believe the answer is:
trees
'dendrologist's study' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trees that I've seen before include "Tall, perennial, woody plants" , "Woods" , "Steer into the plants" , "Tall, perennial, woody plant" , "Long lock of hair" .)