Brit's raincoat (3)
I believe the answer is:
'brit's raincoat' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for mac that I've seen before include "Waterproof coat (colloq.)" , "Overcoat" , "one used in shower?" , "Coat - computer" , "Raincoat (colloq.)" .)