Conjuror's expression (3,6)
I believe the answer is:
hey presto
'conjuror's expression' is the definition.
(expression a conjuror or magician might use)
This is the entire clue.
(Other definitions for hey presto that I've seen before include "Spell words" , "Conjurer's exclamation" , "revealing remark?" , "tricky words?" , "that's the trick" .)