The most common and most likely answer to this crossword clue is the 14 letter word WORCESTERSHIRE
I've seen this crossword clue in Financial Times
W
O
R
C
E
S
T
E
R
S
H
I
R
E
Buddy explains!
The straight part
We know that "where triennial choir festival staged" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where triennial choir festival staged = worcestershire.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.