Scot's ancestor (4)
I believe the answer is:
celt
'scot's ancestor' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for celt that I've seen before include "Early Briton" , "Ancient Briton" , "Pre-Roman inhabitant of Ireland and Britain" , "one who spoke Manx, for example" , "Early European" .)