First of birches in mature or developing tree garden (9)
I believe the answer is:
Here is my best explanation:
'tree garden' is the definition.
(I've seen this before)
'first of birches in mature or developing' is the wordplay.
'first of' says to take the initial letters.
'in' indicates putting letters inside.
'developing' is an anagram indicator.
The initial letter of 'birches' is 'b'.
'matureor' anagrammed gives 'aroretum'.
'b' put into 'aroretum' is 'ARBORETUM'.
(Other definitions for arboretum that I've seen before include "there's high growth", "area with great growth", "Place for cultivation of trees", "Botanical garden of trees", "Garden where trees and shrubs are cultivated", "Botanical tree garden", "Trees and shrubs are cultivated here", "Tree garden", "Tree display centre" and "Collection of elders, say".)