Chit-chat (5,4)
'chit-chat' is the definition.
(I know that chit-chat can be written as small talk)
This is the entire clue.
(Other definitions for small talk that I've seen before include "Polite conversation on trivial matters" , "Unimportant prattle" , "Social chat on unimportant matters" , "Light conversation" , "chattering" .)
(Another definition for idle talk that I've seen is " It's foolish to say".)
(Other definitions for gas that I've seen before include "Form of fossil fuel" , "Talk excessively" , "THIRD PART OF 23" , "Talk idly; fuel" , "Nitrogen maybe" .)