The most common and most likely answer to this crossword clue is the 3 plus 3 plus 7 letter word BOB THE BUILDER
B
O
B
T
H
E
B
U
I
L
D
E
R
Buddy explains!
The straight part
We know that "Artist no.1 at Xmas/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/artist no.1 at xmas = bob the builder.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.