A form of transport by sea or on land (3)
I believe the answer is:
ass
'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" .)