Decoration's not completely exotic in gastronomy (5)
I believe the answer is:
icing
'decoration's' is the definition.
(icing is a kind of decoration)
'not completely exotic in gastronomy' is the wordplay.
'not completely' indicates a hidden word.
'ICING' is hidden in the letters of 'exotic in gastronomy'.
(Other definitions for icing that I've seen before include "Food coating" , "Sugar coating as cake decoration" , "You could see this on a cake" , "white coat" , "Sugar-coating" .)