Scottish surname starter (3)
I believe the answer is:
'scottish surname starter' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mac that I've seen before include "Waterproof raincoat, in short" , "Waterproof coat (colloq.)" , "PC" , "laptop?" , "A Gaelic son, often in name" .)