The most common and most likely answer to this crossword clue is the 7 letter word BALFOUR
B
A
L
F
O
U
R
More clues leading to the result BALFOUR
- Labour in disarray … entertaining loud PM
- Labour: ‘Foot could be PM‘
- Party put up figure to be the PM
- Labour fails to suppress female PM
- Number giving support to revolutionary Labour prime minister
- On the square is rising socialist Prime Minister
- Labour in disarray ... entertaining loud PM
- Labour: 'Foot could be PM'
- PM‘s party almost expressed warning
Buddy explains!

The straight part
We know that "PM" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pm = balfour.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.