Tom's little boat (3)
I believe the answer is:
cat
'boat' is the definition.
(short for catboat)
'tom's little' is the wordplay.
'tom's' becomes 'cat's' (a tom is a male cat).
'little' means to remove the last letter (the word is not quite complete).
'cats' with its last letter taken away is 'CAT'.
(Other definitions for cat that I've seen before include "Feline mammel" , "eg Leopard" , "Grimalkin" , "Moggy" , "Tiger, for instance" .)