Birds’ enclosure originally accessed by means of railway (6)
I believe the answer is:
aviary
'birds' enclosure' is the definition.
'originally accessed by means of railway' is the wordplay.
'originally' suggests taking the first letters.
'by means of' becomes 'via' (I've seen this before).
'railway' becomes 'ry' (abbreviation for railway).
The first letter of 'accessed' is 'a'.
'a'+'via'+'ry'='AVIARY'
(Other definitions for aviary that I've seen before include "Bird sanctuary" , "Large birdcage" , "Cut gem; small film part" , "Its inhabitants probably like flying" , "cages perhaps" .)