Among which ours comes third, but isn't included in seven, musically (7)
I believe the answer is:
planets
'isn't included in seven musically' is the definition.
I can't judge whether this defines the answer.
'among which ours comes third but' is the wordplay.
I cannot really understand how this works, but
'third but' could be 't' (3rd letter of 'but') and 't' is located in the answer.
The remaining letters 'planes' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for planets that I've seen before include "Heavenly bodies" , "Celestial bodies" , "Ten laps (anag.)" , "Big bodies" .)