A Roman road and railway used by fliers (6)
I believe the answer is:
'a roman road and railway used by fliers' is the definition.
I can't judge whether this defines the answer.
'a roman road and railway used by' is the wordplay.
'roman road' becomes 'via' (I've seen this before).
'and' is a charade indicator (letters next to each other).
'railway' becomes 'ry' (abbreviation).
'used by' says to put letters next to each other.
Can you help me to learn more?
(Other definitions for aviary that I've seen before include "Bird-house" , "Bird enclosure" , "Its inhabitants probably like flying" , "wildlife centre" , "Large bird cage" .)