First of birches in mature or developing tree garden (9)
I believe the answer is:
arboretum
'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'.
'mature'+'or'='matureor'
'matureor' anagrammed gives 'aroretum'.
'b' put into 'aroretum' is 'ARBORETUM'.
(Other definitions for arboretum that I've seen before include "there's high growth" , "Botanical garden of trees" , "Where planes may be" , "Garden where trees and shrubs are cultivated" , "Botanical tree garden" .)