Mechanical model of the solar system (6)
I believe the answer is:
orrery
'mechanical model of the solar system' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for orrery that I've seen before include "Clockwork model of the sun and planets" , "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" .)