The most common and most likely answer to this crossword clue is the 12 letter word MASTERPIECES
I've seen this crossword clue in The Sunday Telegraph
M
A
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More clues leading to the result MASTERPIECES
Buddy explains!
The straight part
We know that "Best work" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case best work = masterpieces.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 MASTERPIECES mean?
MASTERPIECES noun- A piece of work that has been given much critical praise, especially one that is considered the greatest work of a person's career.
- A work of outstanding creativity, skill or workmanship.
- A work created in order to qualify as a master craftsman and member of a guild.