Talk of nitrogen, 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)
'nitrogen say' is the second definition.
(nitrogen is a kind of gas)
'of' acts as a link.
(Other definitions for gas that I've seen before include "that may quickly be stepped on" , "Hydrogen possibly" , "idle chit-chat" , "Something exciting" , "Possibly noble" .)