Descent; melody (6)
I believe the answer is:
strain
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">descent</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >'<span class="explanation-format__original">melody</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that melody can be written as strain)</span><br/ ><br/ ></span>
(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" .)