The most common and most likely answer to this crossword clue is the 11 letter word MARKETPLACE
I've seen this crossword clue in The Daily Telegraph
M
A
R
K
E
T
P
L
A
C
E
More clues leading to the result MARKETPLACE
Buddy explains!

The straight part
We know that "Billingsgate?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case billingsgate? = marketplace.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 MARKETPLACE mean?
MARKETPLACE noun- An open area in a town housing a public market.
- The space, actual or metaphorical, in which a market operates.
- (by extension) The world of commerce and trade.
- A place or sphere for the exchange of anything, such as ideas or fashions.