The most common and most likely answer to this crossword clue is the 3 plus 4 plus 1 plus 4 plus 4 letter word THE MAGI - THEM AGIN
I've seen this crossword clue in The Times
T
H
E
M
A
G
I
-
T
H
E
M
A
G
I
N
Buddy explains!
The straight part
We know that "Seasonal visitors" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seasonal visitors = the magi - them agin.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.