Stress line of descent (6)
I believe the answer is:
strain
I believe this is a triple definition!
'stress' is the first definition.
(synonyms)
'line' is the second definition.
(synonyms)
'of' is a link.
'descent' is the third definition.
(thesaurus)
(Other definitions for strain that I've seen before include "Blood" , "Tension - stock" , "song" , "Wrench; melody" , "Noted" .)