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