Then, a goldfish flying! Conjuror's trickery (7,2,4)
I believe the answer is:
sleight of hand
'conjuror's trickery' is the definition.
(technique used by a magician)
'then a goldfish flying' is the wordplay.
'flying' indicates anagramming the letters.
'then'+'a'+'goldfish'='thenagoldfish'
'thenagoldfish' anagrammed gives 'SLEIGHT-OF-HAND'.
(Other definitions for sleight of hand that I've seen before include "Conjuror's skill" , "Legerdemain" , "Prestidigitation" , "Doctor or student doctor" , "Trickery" .)