Mole in railway carriage (7)
I believe the answer is:
sleeper
'carriage' is the definition.
(sleeper is a kind of carriage)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Brace to support rail on track" , "film not successful at first" , "Wagon-lit" , "Unactivated spy - railway support" , "One who slumbers" .)