Balloon gas (6)
I believe the answer is:
helium
'balloon gas' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for helium that I've seen before include "Balloon-filling element" , "Light colourless gas used in airships" , "Second lightest element" , "Lightest noble gas" , "Light colourless gas used in balloons" .)