Cake decoration set in an art institute (8)
I believe the answer is:
Here is my best explanation:
'cake decoration' is the definition.
(I've seen this before)
'set in an art institute' is the wordplay.
'set' becomes 'gel' (I've seen this before).
'in' is an insertion indicator.
'art institute' becomes 'ica' (Institute of Contemporary Arts).
'gel' going within 'anica' is 'ANGELICA'.
(Other definitions for angelica that I've seen before include "Its candied leaves are used in cake decoration", "Candied plant stem", "Herb", "Candied stem, decorates cake", "Aromatic plant", "Candied stems of a plant", "Nag Alice to get fragrant plant", "Candied stalk used in cake decoration", "Wild plant, often crystallised" and "Candied stalks used to decorate cakes etc".)