Descending line in melody (6)
I believe the answer is:
strain
This might be a double definition.
'descending line' is the first definition.
'strain' can be an answer for 'line' (I have seen 'line of ancestry' mean 'strain' so perhaps 'line' could also mean 'strain'). I'm not certain of the 'descending' bit.
'melody' is the second definition.
(I've seen this before)
'in' is the link.
Can you help me to learn more?
(Other definitions for strain that I've seen before include "Exert oneself to the utmost" , "Muscle injury - tension" , "Trains under stress and pressure" , "Blood" , "Struggle" .)