The most common and most likely answer to this crossword clue is the 8 letter word GUESTING
I've seen this crossword clue in The Daily Telegraph
G
U
E
S
T
I
N
G
Buddy explains!
The straight part
We know that "What star visitor is doing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what star visitor is doing = guesting.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 GUESTING mean?
GUESTING verb- To appear as a guest, especially on a broadcast
- As a musician, to play as a guest, providing an instrument that a band/orchestra does not normally have in its line up (for instance, percussion in a string band)
- To receive or entertain hospitably.
- An appearance as a guest on a TV show