Drop line goes, argument follows in a shrill voice (8)
I believe the answer is:
falsetto
'drop line goes' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'argument follows in a shrill voice' is the wordplay.
'argument' becomes 'setto' (I've seen this before).
'follows' is a charade indicator (letters next to each other).
'in a shrill voice' becomes 'fal' (I am not sure about this - if you are sure you should believe this answer much more).
'setto' put after 'fal' is 'FALSETTO'.
Can you help me to learn more?
(Other definitions for falsetto that I've seen before include "Let fatso have voice above natural range" , "Singing voice forced above its natural one" , "Artificial pitch" , "Flat toes (anag.)" , "Voice that's raised" .)