Everything one left at the rail terminus (3)
I believe the answer is:
all
'everything' is the definition.
(all things)
'one left at the rail terminus' is the wordplay.
'one left' becomes 'L' (common abbreviation).
'at' means one lot of letters go next to another (I've seen this in other clues).
'the rail terminus' becomes 'al' (I can't explain this - if you can you should give a lot more credence to this answer).
'l' put after 'al' is 'ALL'.
Can you help me to learn more?
(Other definitions for all that I've seen before include "that's as many as there are" , "Entire" , "Passage in a formal garden" , "Everyone - complete" , "The lot, everyone" .)