A mechanical model of the solar system (6)
I believe the answer is:
orrery
'a mechanical model of the solar system' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for orrery that I've seen before include "Mechanical model of the solar system" , "Planetary model" , "Planetarium that illustrates the positions and motions of bodies in the solar system" , "Err, Roy, on the side of a model of the solar system" , "Clockwork model of the solar system" .)