Sauce that’s very nice indeed (7)
I believe the answer is:
topping
I believe this is a double definition.
'sauce' is the first definition.
(sauce is a type of topping)
'very nice indeed' is the second definition.
(topping can mean excellent)
'that's' acts as a link.
(Other definitions for topping that I've seen before include "Garnish - jolly good!" , "Polishing off" , "garnish for food" , "Garnish - splendid!" , "Crowning" .)