First couple heading off assessing cost of decoration (5)
I believe the answer is:
icing
'decoration' is the definition.
(icing is a kind of decoration)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for icing that I've seen before include "causes trouble at airports?" , "in the freezer?" , "it's dangerous on the roads" , "Cake covering" , "You could see this on a cake" .)