Heavenly cake decoration left behind (9)
I believe the answer is:
angelical
'heavenly' is the definition.
(I've seen this before)
'cake decoration left behind' is the wordplay.
'cake decoration' becomes 'angelica' (angelica stalks are used to decorate cakes).
'left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'angelica'+'l'='ANGELICAL'
(Other definitions for angelical that I've seen before include "heavenly" , "sublime innocence" , "Exceptionally beautiful" .)