PC alternative (3)
I believe the answer is:
mac
'pc alternative' 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 "It may help you keep dry" , "PC" , "Coat - computer" , "Waterproof coat (colloq.)" , "A Gaelic son, often in name" .)