(a) musical
(Un) musical is a Spanish term used to indicate a musical show or performance. This term often refers to a genre of drama in which singing and dancing play an essential role, often telling a story. The term is derived from the English word 'musical', and it's used almost universally across the Spanish-speaking world. However, like many cultural terms, the context and usage can vary slightly from region to region, but generally the term '(un) musical' always relates to a show or performance which incorporates music, dancing, and often singing too.
My brother works in a musical on Broadway.
This sentence is using 'un musical' to refer to a type of performance that combines songs, spoken dialogue, acting, and dance. The 'un' definite article is used as 'a' in English.
We are going to see a musical tonight.
In this example, 'un musical' is used to refer to the event that the speaker will attend. The use of 'un' as a definite article implies that it's a single event they will be attending.
She is the star of a famous musical.
This sentence states that the person being talked about is the main actor or performer in a well-known musical. 'Un musical' is being used to identify the type of performance where this individual has a starring role.