One's always travelling, never off the rails (5)
I believe the answer is:
nomad
'one's always travelling' is the definition.
I can't judge whether this defines the answer.
'never off the rails' is the wordplay.
'never' becomes 'no' (I've seen this in other clues).
'off the rails' becomes 'mad' (I can't explain this - if you can you should give a lot more credence to this answer).
'no'+'mad'='NOMAD'
Can you help me to learn more?
(Other definitions for nomad that I've seen before include "one of those with no fixed home" , "Damon (anag.)" , "Migrant" , "One keeps moving" , "Person of no fixed abode" .)