Bird is able to appear on railway (6)
I believe the answer is:
canary
'bird' is the definition.
(canary bird is a kind of bird)
'able to appear on railway' is the wordplay.
'able' becomes 'can'.
'to' is a charade indicator (letters next to each other).
'appear' becomes 'a' (this could be a standard abbreviation which I don't know about).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'railway' becomes 'ry' (abbreviation for railway).
'can'+'a'+'ry'='CANARY'
'is' is the link.
Can you help me to learn more?
(Other definitions for canary that I've seen before include "This flies" , "Pet bird" , "Popular cage bird, often yellow" , "Songbird once used to detect toxic gases in mines" , "grass" .)