The most common and most likely answer to this crossword clue is the 12 letter word BACKBENCHERS
I've seen this crossword clue in The Daily Telegraph
B
A
C
K
B
E
N
C
H
E
R
S
More clues leading to the result BACKBENCHERS
Buddy explains!
The straight part
We know that "MPs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mps = backbenchers.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 BACKBENCHERS mean?
BACKBENCHERS noun- A Member of Parliament who does not have cabinet rank, and who therefore sits on one of the backbenches or in one of the back rows of the legislature.
- A student who does not perform well, especially one who sits at the back of the classroom.
- A member of a team who does not usually play, but who is held in reserve.
- (by extension) Someone who does not play an active role in a process.