They fly or take a train (5)
I believe the answer is:
rails
'they fly' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'take a train' is the wordplay.
'take' becomes 'r' (abbreviation for 'recipe', Latin for 'take').
'a train' becomes 'ails' (I can't justify this - if you can you should believe this answer much more).
'r'+'ails'='RAILS'
'or' acts as a link.
Can you help me to learn more?
(Other definitions for rails that I've seen before include "Metal bars of a barrier" , "Makes a protest" , "Water birds" , "Wading birds - dysfunctional person has come off them" , "Barriers of horizontal bars and supports" .)