A blow in third argument was disconcerting (8)
I believe the answer is:
abashing
'disconcerting' is the definition.
(abashing is a kind of disconcerting)
'a blow in third argument' is the wordplay.
'blow' becomes 'bash' (bash is a kind of blow).
'third argument' becomes 'g' (3rd letter of 'argument').
'a'+'bash'+'in'+'g'='ABASHING'
'was' is the link.
(Other definitions for abashing that I've seen before include "humiliating" , "Causing to feel self-conscious" , "embarrassing" , "disconcerting" .)