Descent; melody (6)
I believe the answer is:
strain
This clue is a double definition.
'descent' is the first definition.
(thesaurus)
'melody' is the second definition.
(I know that melody can be written as strain)
(Other definitions for strain that I've seen before include "Pass through a sieve" , "Sift" , "Put through a perforated device to separate" , "Stretch or exert to the utmost" , "Strive; ancestry" .)