Sub-postmaster against 3 in film sat happily with beer (6,5)

I believe the answer is:

mister bates

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

'sub-postmaster against 3' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'film sat happily with beer' is the wordplay.
I cannot quite understand how this works, but
'film' could be 'mist' (to film over is to mist or fog over) and 'mist' is found in the answer.
This explanation may well be incorrect...

'in' acts as a link.

Can you help me to learn more?

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.