Talk of helium, say (3)
I believe the answer is:
gas
This clue is a double definition.
'talk' is the first definition.
(gas can mean to chat)
'helium say' is the second definition.
(helium is a kind of gas)
'of' is the link.
(Other definitions for gas that I've seen before include "Neon, for example" , "Nitrogen, say" , "chew the fat" , "Fumes, vapour" , "Eg, oxygen" .)