A ship is a form of transport (3)
I believe the answer is:
ass
'form of transport' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'a ship' is the wordplay.
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'a'+'ss'='ASS'
'is a' is the link.
Can you help me to learn more?
(Other definitions for ass that I've seen before include "Pompous person" , "Charlie" , "jack" , "Buffoon" , "A donkey or an idiot" .)