The most common and most likely answer to this crossword clue is the 10 letter word FESTOONING
I've seen this crossword clue in Independent
F
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Buddy explains!

The straight part
We know that "Decorating" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case decorating = festooning.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 FESTOONING mean?
FESTOONING verb- To decorate with ornaments, such as garlands or chains, which hang loosely from two tacked spots.
- To make festoons.
- To decorate or bedeck abundantly.
- To apply texturing to (a denture) to simulate human tissue.
- Material with which something is festooned.