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' says to put letters next to each other (I've seen this in other clues).
'the rail terminus' becomes 'al' (I can't justify 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" .)