Unexpected error over variable astronomical model (6)
I believe the answer is:
orrery
'astronomical model' is the definition.
'orrery' can be an answer for 'model' (orrery is a kind of model). I'm unsure of the 'astronomical' bit.
'unexpected error over variable' is the wordplay.
'unexpected' indicates anagramming the letters.
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'variable' becomes 'y' (algebra).
'error' anagrammed gives 'orrer'.
'orrer'+'y'='ORRERY'
Can you help me to learn more?
(Other definitions for orrery that I've seen before include "Planetary model" , "Clockwork model of the sun and planets" , "Mechanical model of the solar system" , "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" .)