Involuntary expulsion of air (6)
I believe the answer is:
sneeze
'involuntary expulsion of air' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sneeze that I've seen before include "reason for blessing" , "Explosive nasal action" , "Precursor to "bless you"" , "Sudden involuntary expulsion of air" , "To which gesundheit is often a response" .)