The most common and most likely answer to this crossword clue is the 12 letter word OFF-BROADWAY
O
F
F
-
B
R
O
A
D
W
A
Y
Buddy explains!
The straight part
We know that "an experimental theatre?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an experimental theatre? = off-broadway.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.