Sack from the Waterloo train (4)
I believe the answer is:
loot
'sack' is the definition.
(I've seen this before)
'from the waterloo train' is the wordplay.
'from' says the answer is hidden in the clue.
'LOOT' is hidden in the letters of 'waterloo train'.
(Other definitions for loot that I've seen before include "Steal property (in war)" , "Burglar's takings - pillage" , "Orton play; ill-gotten gains" , "Swag, booty" , "Steal (goods); money" .)