Railway supporter is a secret agent (7)
I believe the answer is:
sleeper
'secret agent' is the definition.
(I know that sleeper is a type of spy)
'railway supporter is a' is the wordplay.
'railway supporter' becomes 'slee' (I can't explain this - if you can you should give a lot more credence to this answer).
'is a' becomes 'per' (eg 'one a day' means 'one per day').
'slee'+'per'='SLEEPER'
Can you help me to learn more?
(Other definitions for sleeper that I've seen before include "Long-inactive secret agent" , "Form of transport" , "Supportive log on railway track" , "Support for railway lines" , "Railway track support" .)