The sound of a sneeze (7)
I believe the answer is:
atishoo
'the sound of a sneeze' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for atishoo that I've seen before include "Sound made on expiration" , "Word to be sneezed at" , "report of involuntary escape from trap?" , "Sudden eruption" , "Sneezing sound" .)