Excellent form! (5)
I believe the answer is:
class
This clue is a double definition.
'excellent' is the first definition.
(I've seen this before)
'form' is the second definition.
(I know that form can be written as class)
(Other definitions for class that I've seen before include "Rank" , "Order - quality" , "seminar" , "Genus" , "Form; stylishness" .)