When to make a start as PM? (2,4)

I believe the answer is:

at noon

I'm a little stuck... Click here to teach me more about this clue!

'start as pm?' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'when to make a' is the wordplay.
I cannot quite see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
'a' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.