Apprentice, one on the railways? (7)
I believe the answer is:
trainee
'apprentice' is the definition.
(someone undergoing training)
'one on the railways?' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' means one lot of letters goes inside another.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'railways?' becomes 'ranee' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'ranee'='tranee'
'i' put into 'tranee' is 'TRAINEE'.
Can you help me to learn more?
(Other definitions for trainee that I've seen before include "A beginner in Aintree" , "One learning a job" , "Person being taught" , "one not yet qualified" , "Person being instructed in work" .)