First-class food garnish (7)
I believe the answer is:
topping
This could be a double definition.
'first-class' is the first definition.
(old-fashioned term meaning excellent)
'food garnish' is the second definition.
'topping' can be an answer for 'garnish' (topping is a kind of garnish). I'm not certain of the 'food' bit.
Can you help me to learn more?
(Other definitions for topping that I've seen before include "Garnish - splendid!" , "murder" , "Sauce; excellent (dated)" , "A1" , "Excellent; eg, icing" .)