Undercover agent in railway carriage (7)
I believe the answer is:
sleeper
I believe this is a quadruple definition!!
'undercover' is the first definition.
(I've seen this before)
'agent' is the second definition.
(a sleeper is a type of secret agent)
'in' acts as a link.
'railway' is the third definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'carriage' is the fourth definition.
(sleeper is a kind of carriage)
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Possibly ring" , "Earring" , "Supporting block on railway track" , "Undercover" , "Supportive log on railway track" .)
