Learn Portuguese (Portugal): Ele não é um palhaço velho, mas é um velho palhaço no circo. - He is not an old clown, but he is a longtime clown in the circus.

In Portuguese, placing certain adjectives before the noun makes them more subjective or figurative, while placing them after makes them literal and objective. Palhaço velho (after) literally means his physical age is old (an elderly clown). Velho palhaço (before) is figurative, meaning he has been a clown for a long time, regardless of his actual age.

Normally we don't use um with professions, so why does this sentence say um palhaço instead of just ele não é palhaço velho?

You are right that Portuguese usually drops the article for professions (like ele é palhaço). However, when you add an adjective to describe the profession (like velho here), you must bring the article back. That is why it has to be um palhaço velho.

Could I say um antigo palhaço instead of um velho palhaço to mean a longtime clown?

Yes, but they feel slightly different. Um antigo palhaço often means a former clown (someone who used to do it but stopped). Um velho palhaço focuses more on the fact that he has been doing it for many years, and usually implies he is still active in the profession.

How is mas é pronounced here? Does mas sound like mais?