The most common and most likely answer to this crossword clue is the 9 letter word MACMILLAN
I've seen this crossword clue in The Times
M
A
C
M
I
L
L
A
N
More clues leading to the result MACMILLAN
- Old PM’s official staff shortened press article
- Coming from Eton, all I’m campaigning about is old boy making it as PM
- Chap keeping Duchess of Cornwall apart from a former PM
- Tory prime minister in decline, contributing to damn all, I’m Cameron!
- Fellow keeping a factory cold inside
- Famous Blue
- Fellow embracing Prince’s wife? Not a prime minister!
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 = macmillan.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.