The most common and most likely answer to this crossword clue is the 3 plus 6 letter word BOX OFFICE
I've seen this crossword clue in The Times
B
O
X
O
F
F
I
C
E
Buddy explains!
The straight part
We know that "Ticket seller" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ticket seller = box office.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 BOX OFFICE mean?
BOX OFFICE noun- A place where tickets are sold in a theatre/theater or cinema.
- (by extension) the total amount of money paid by people worldwide to watch a movie at cinemas/movie theaters.