Vehicles carrying a heavy weight in containers (7)

I believe the answer is:

cartons

'containers' is the definition.
(carton is a kind of container)

'vehicles carrying a heavy weight' is the wordplay.
'vehicles' becomes 'cars' (car is a kind of vehicle).
'carrying' is an insertion indicator.
'a heavy weight' becomes 'ton' (I've seen this before).
'cars' enclosing 'ton' is 'CARTONS'.

'in' is the link.

(Another definition for cartons that I've seen is " Cardboard containers".)

I've seen this clue in The Globe and Mail.

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