Balloon filler (3,3)
I believe the answer is:
hot air
'balloon filler' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for hot air that I've seen before include "Gas" , "is essential before the balloon goes up" , "Bluster" , "Loud and empty talk to lift a balloon?" , "Talking nonsense" .)