In this, one truly was in the cart (7)
I believe the answer is:
tumbrel
'cart' is the definition.
(tumbrel is a kind of cart)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tumbrel that I've seen before include "Belt rum (anag.); prisoners' transport" , "Kind of cart that once brought aristocrats to the guillotine" , "Cart in which aristocrats went to the guillotine" , "transport to block" , "prisoner transport" .)