The most common and most likely answer to this crossword clue is the 5 plus 3 letter word BONAR LAW
I've seen this crossword clue in The Sunday Telegraph
B
O
N
A
R
L
A
W
More clues leading to the result BONAR LAW
Buddy explains!
The straight part
We know that "old PM" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old pm = bonar law.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.