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' acts as a 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" .)