Would a tartan lining suit it? (3)
I believe the answer is:
mac
If this is the answer I cannot explain how the clue works.
Can you help me to learn more?
(Other definitions for mac that I've seen before include "Waterproof coat (colloq.)" , "It may help you keep dry" , "Overcoat" , "A rainproof" , "Coat - computer" .)