Cryptic crossword clue
As for No 10 being reformed, PM starts with these 10

The most common and most likely answer to this crossword clue is the 10 letter word AFTERNOONS

I've seen this crossword clue in The Guardian
A
F
T
E
R
N
O
O
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S

Buddy explains!

The straight part
We know that "PM starts with these" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pm starts with these = afternoons.
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 AFTERNOONS mean?

AFTERNOONS noun
  1. The part of the day from noon or lunchtime until sunset, evening, or suppertime or 6pm.
  2. The later part of anything, often with implications of decline.
  3. A party or social event held in the afternoon.

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