Excellent finishing, not starting (7)
I believe the answer is:
topping
'excellent' is the definition.
(I've seen this before)
'finishing not starting' is the wordplay.
'finishing' becomes 'stopping' ('stop' can be a synonym of 'finish').
'not starting' means to remove the first letter.
'stopping' with its initial letter removed is 'TOPPING'.
(Other definitions for topping that I've seen before include "Absolutely excellent!" , "Crowning" , "Sauce; excellent (dated)" , "custard?" , "murder" .)