The most common and most likely answer to this crossword clue is the 8 letter word FROSTING
I've seen this crossword clue in Financial Times
F
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Buddy explains!
The straight part
We know that "topping" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case topping = frosting.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does FROSTING mean?
FROSTING verb- To cover with frost.
- To become covered with frost.
- To coat (something, e.g. a cake) with icing to resemble frost.
- To anger or annoy.
- To sharpen (the points of a horse's shoe) to prevent it from slipping on ice.
- A sugary coating for cakes and other baked goods.
- A layer of frost.
- The theft of a car while it is left unattended, especially when its engine is left running in the winter to defrost the car.