The most common and most likely answer to this crossword clue is the 5 letter word SLADE
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A
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Buddy explains!

The straight part
We know that "artist no.1 at Xmas" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case artist no.1 at xmas = slade.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 SLADE mean?
SLADE noun- A valley, a flat grassy area, a glade.
- The sole of a plough.
- A spade for digging peat.