not a teacher
blown-in-the-glass = genuine, authentic, trustworthy.
blown-in-the-glass : Green's Dictionary of Slang Oxford Reference
clincher = a fact, argument or event that is decisive or settles a matter.
clincher - definition. American English definition of clincher by Macmillan Dictionary
a blown-in-the-glass clincher = something that is genuinely decisive and settles a matter, for example an argument or contest of some sort, or ensures a success.