The most common and most likely answer to this crossword clue is the 13 letter word ORNAMENTATION
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More clues leading to the result ORNAMENTATION
Buddy explains!

The straight part
We know that "garnish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case garnish = ornamentation.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 ORNAMENTATION mean?
ORNAMENTATION noun- Decoration, adornment or embellishment.
- The act or process of decorating etc.
- Short notes added to a composition to emphasize certain notes and to add style.