Botanical tree garden (9)
I believe the answer is:
arboretum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">botanical 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 "Place for cultivation of 19s" , "Botanical tree garden" , "Botanical garden of trees" , "Collection of elders, say" , "area with great growth" .)