The most common and most likely answer to this crossword clue is the 1 plus 1 plus 1 plus 4 letter word V O C PARK
V
O
C
P
A
R
K
Buddy explains!
The straight part
We know that "a public venue in 1 Dn. and 15 Ac." is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a public venue in 1 dn. and 15 ac. = v o c park.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.