Butler juggling with claret — it's trajectory is clearly visible (6,6)

I believe the answer is:

tracer bullet

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

'butler' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

I've seen this clue in The Guardian.

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.