There's nothing in the instrument but a crown (7)
I believe the answer is:
coronet
'a crown' is the definition.
(I know that coronet is a type of crown)
'there's nothing in the instrument' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in the' means one lot of letters goes inside another.
'instrument' becomes 'cornet' (cornet is a kind of instrument).
'o' put into 'cornet' is 'CORONET'.
'but' acts as a link.
(Other definitions for coronet that I've seen before include "Small crown" , "Headwear" , "topping item of jewellery" , "badge of rank" , "sign of rank" .)