Bird enclosure principally approached by public transport (6)
I believe the answer is:
aviary
'bird enclosure' is the definition.
(an aviary is a large enclosure for birds)
'principally approached by public transport' is the wordplay.
'principally' suggests taking the first letters.
'by' becomes 'via' (travelling by or via a place).
'public transport' becomes 'ry' (abbreviation for railway).
The initial letter of 'approached' is 'a'.
'a'+'via'+'ry'='AVIARY'
(Other definitions for aviary that I've seen before include "House for birds" , "Bird enclosure" , "Building for keeping birds" , "Large birdcage" , "Large enclosure for birds" .)