One to embrace decisive argument (8)
I believe the answer is:
clincher
I believe this is a double definition.
'one to embrace' is the first definition.
(someone who clinches or embraces)
'decisive argument' is the second definition.
(I've seen this before)
(Other definitions for clincher that I've seen before include "Argument-settling fact" , "Something settling a matter conclusively" , "Something decisive" , "Decisive argument" , "Decisive factor" .)