The most common and most likely answer to this crossword clue is the 14 letter word OPISTHODOMOSES
O
P
I
S
T
H
O
D
O
M
O
S
E
S
Buddy explains!
The straight part
We know that "Back rooms" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case back rooms = opisthodomoses.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.