Botanical tree garden (9)
I believe the answer is:
arboretum
'botanical tree garden' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for arboretum that I've seen before include "Place for cultivation of 19s" , "Botanical tree garden" , "Botanical garden of trees" , "Collection of elders, say" , "area with great growth" .)