Of direct descent (6)
I believe the answer is:
lineal
'of direct descent' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lineal that I've seen before include "In a line as, say, a descendant" , "One-dimensional" , "In a straight row" , "In line of descendants" , "Having only one dimension" .)