(drama, film) a song sung between acts or similar breaks; an intermezzo / (anime) insert song; a piece of incidental music

quack doctor

necessity (quality or state of being necessary, unavoidable, or absolutely requisite)

unnecessary

needlessness

necessary evil

having a comeback or excuse for everything one is told, being smart in the sense of having a smart mouth

(idiomatic) just as [someone] says

as a matter of fact; to tell the truth

saying as much as one wants (usually complaints)