Almost drop argument for register of singers (8)
I believe the answer is:
falsetto
'register of singers' is the definition.
'falsetto' can be an answer for 'register' (I've seen this before). I'm unsure of the 'of singers' bit.
'almost drop argument for' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'drop' becomes 'fall' (synonyms).
'argument for' becomes 'setto' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fall' with its last letter removed is 'fal'.
'fal'+'setto'='FALSETTO'
Can you help me to learn more?
(Other definitions for falsetto that I've seen before include "Abnormally high voice" , "Artificially high voice above the natural range" , "it sounds high" , "High-pitched male voice" , "high notes?" .)