Tree garden (9)
I believe the answer is:
arboretum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tree garden</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for arboretum that I've seen before include "area with great growth" , "Where planes may be" , "Botanical tree garden" , "there's high growth" , "Garden where trees and shrubs are cultivated" .)