The most common and most likely answer to this crossword clue is the 9 letter word REPERTORY
I've seen this crossword clue in The Daily Telegraph
R
E
P
E
R
T
O
R
Y
More clues leading to the result REPERTORY
Buddy explains!

The straight part
We know that "Theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case theatre = repertory.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 REPERTORY mean?
REPERTORY noun- A repertoire
- A collection of things, or a place where such a collection is kept
- A specific set of works that a company performs
- A theater in which a resident company presents works from a specified repertoire, usually in alternation.
- A repertory company.