Separate line of descent (6)
I believe the answer is:
strain
This clue is a triple definition!
'separate' is the first definition.
(straining is a kind of separating)
'line' is the second definition.
('strain' can be a synonym of 'line')
'of' acts as a link.
'descent' is the third definition.
(thesaurus)
(Other definitions for strain that I've seen before include "Stress or make taut" , "making a great demand on the emotions?" , "Tension - stock" , "In arts (anag)" , "Remove by passing through a filter" .)