The most common and most likely answer to this crossword clue is the 6 plus 4 letter word BUCKET SHOP
I've seen this crossword clue in Financial Times
B
U
C
K
E
T
S
H
O
P
Buddy explains!
The straight part
We know that "A not wholly reliable ticket agency" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a not wholly reliable ticket agency = bucket shop.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.