They transport canisters from which contents have been lost (4)
I believe the answer is:
cars
'they' is the definition.
Both the definition and answer are plural nouns.
Maybe they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for cars that I've seen before include "... when these may be driven?" , "Automobiles" , "Private vehicles" , "Motor vehicles" .)