Model of the solar system
I believe the answer is:
'model of the solar system' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for orrery that I've seen before include "Clockwork model of the solar system" , "Mechanical model of the solar system" , "Err, Roy, on the side of a model of the solar system" , "Planetarium that illustrates the positions and motions of bodies in the solar system" , "Clockwork model of the sun and planets" .)