Magicians missing trick? There are a dozen in a box (6)
I believe the answer is:
jurors
'there are a dozen in a box' is the definition.
(12 jurors sit in the jury box in a courtroom)
'magicians missing trick?' is the wordplay.
'magicians' becomes 'conjurors' (conjuror is a kind of magician).
'missing' is a deletion indicator.
'trick?' becomes 'con' (I've seen this before).
'conjurors' with 'con' removed is 'JURORS'.
(Other definitions for jurors that I've seen before include "Twelve citizens in court" , "Court adjudicators" , "We hear a case" , "box contains a dozen" .)