Talk of argon, say (3)
I believe the answer is:
gas
I believe this clue is a double definition.
'talk' is the first definition.
(gas can mean to chat)
'argon say' is the second definition.
(argon is a kind of gas)
'of' acts as a link.
(Other definitions for gas that I've seen before include "Eg, nitrogen" , "Talk idly; fuel" , "Talk profusely" , "Idle chat" , "Hydrogen or oxygen, say" .)