Cake decoration, mostly sweet (7)
I believe the answer is:
angelic
'sweet' is the definition.
(I know that sweet can be written as angelic)
'cake decoration mostly' is the wordplay.
'cake decoration' becomes 'angelica' (angelica stalks are used to decorate cakes).
'mostly' means to remove the last letter.
'angelica' with its final letter taken away is 'ANGELIC'.
(Other definitions for angelic that I've seen before include "Of innocent beauty" , "The host's" , "Sweet and well-behaved" , "Payment for release" , "Extremely well-behaved" .)