Acted as stand-in and did a dentist's job (6,3,3)

I believe the answer is:

filled the gap

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

But I can't explain it at all! Could you help me understand?

I've seen this clue in The Globe and Mail.

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.