A form of transport by sea or on land (3)

I believe the answer is:

ass

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'a form' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for ass that I've seen before include "Stupid person or American bum" , "Stupid fellow" , "Dipstick" , "Braying creature" , "animal inside" .)

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

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