Stop here possibly to take in variable 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)
'stop here possibly to take in variable' is the wordplay.
'possibly' indicates anagramming the letters.
'to take in' means one lot of letters goes inside another.
'variable' becomes 'y' (algebra).
'stop'+'here'='stophere'
'stophere' with letters rearranged gives 'hepresto'.
'hepresto' enclosing 'y' is 'HEY-PRESTO'.
(Other definitions for hey-presto that I've seen before include "The trick is done!" , "Conjuror's cry" , "Top heresy about magician's cry" , "Abracadabra" , "Conjurer's words at key moment" .)