A little machine that's waterproof (3)
I believe the answer is:
mac
'waterproof' is the definition.
(mac is a kind of waterproof)
'a little machine that's' is the wordplay.
'a little' says to take the initial letters.
'that's' becomes 'ac' (I can't explain this - if you can you should give a lot more credence to this answer).
The first letter of 'machine' is 'm'.
'm'+'ac'='MAC'
Can you help me to learn more?
(Other definitions for mac that I've seen before include "Waterproof raincoat, in short" , "Waterproof coat (colloq.)" , "It may help you keep dry" , "laptop?" , "PC" .)