Chopping up topless 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 "Possibly the nicest part of the cake" , "Deposit" , "Formation of frost on aircraft" , "Sweet cake topping" , "Sugar-coating" .)