Whoopee cushion filler (3)
I believe the answer is:
air
'whoopee cushion filler' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for air that I've seen before include "- - on a G-string" , "Outward appearance" , "A tune or attitude" , "Spread publicity" , "Bearing, manner" .)