It's the whole phrase "loud and clear" that is idiomatic. It means unequivocal(ly), emphatic(ally), unambiguous(ly).
It can refer to a spoken communication: "When the teacher scolded the students for their bad behaviour, the message came across loud and clear."
Or some other form: "The protests were loud and clear in the community newspaper for several weeks following the closure of the library."
not a teacher
Student or Learner