Impervious to argument about half of deaths, full stop! (4)
I believe the answer is:
deaf
'impervious' is the definition.
(thesaurus)
'argument about half of deaths full stop' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found in the answer.
The remaining letters 'def' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for deaf that I've seen before include "Unhearing" , "Unable to pick up" , "Receiving no sound" , "Rudely confident" , "Aurally challenged" .)