The most common and most likely answer to this crossword clue is the 6 plus 8 letter word PUBLIC SPEAKING
I've seen this crossword clue in The Guardian
P
U
B
L
I
C
S
P
E
A
K
I
N
G
Buddy explains!
The straight part
We know that "something MPs must get used to" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something mps must get used to = public speaking.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.