One lying about being run over by a train? (7)
I believe the answer is:
sleeper
'one lying about being run' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'by a train?' is the wordplay.
'by' says to put letters next to each other.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'train?' becomes 'slee' (I can't explain this - if you can you should believe this answer much more).
'per' after 'slee' is 'SLEEPER'.
'over' is the link.
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Long-inactive secret agent" , "One who slumbers" , "Rail track supporter" , "Rail support; enemy spy" , "Brace to support rail on track" .)