Bubble filler (3)
I believe the answer is:
air
'bubble filler' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for air that I've seen before include "Ventilate or give expression to" , "Ventilate, say grievance" , "- brick; - bed; - brush" , "strain" , "14" .)