What sometimes follows royal decoration (5)
I believe the answer is:
icing
'decoration' is the definition.
(icing is a kind of decoration)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for icing that I've seen before include "Maybe Madeira's topping" , "Sweet thing" , "piece of cake?" , "Sugary topping" , "Freezing" .)