The most common and most likely answer to this crossword clue is the 5 plus 7 letter word BRAIN SURGEON
I've seen this crossword clue in Financial Times
B
R
A
I
N
S
U
R
G
E
O
N
More clues leading to the result BRAIN SURGEON
Buddy explains!
The straight part
We know that "one working in theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one working in theatre = brain surgeon.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.