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 subsidiary indication.
'first of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'developing' indicates anagramming the letters.
The initial letter of 'birches' is 'b'.
'matureor' with letters rearranged gives 'aroretum'.
'b' inserted into 'aroretum' is 'arboretum'.
(Other definitions for arboretum that I've seen before include "Botanical garden of trees", "Tree display centre", "Garden where trees and shrubs are cultivated", "Botanical tree garden", "Tree garden", "Place for cultivation of trees", "Trees and shrubs are cultivated here", "Botanic garden of trees" and "Place for cultivation of 19s".)