Former military or police who inflate their accomplishments and bring a giant ego to their next job, especially in training. After leaving the military, Tier-One Ass-Clowns put in the role of instructor of weapon and tactics usually endanger students with a toxic mix of their own ego and flawed training doctrine.
Ben: What did Ralph and Bert do in the military before they came to training?
Curtis: Well I know one was military police and I think other was in Special Forces but worked "at the rear with the gear" instead of forward deployed.
A person who feels the need to instruct others how to perform better at something, despite lacking the background to do so or the ability to translate their knowledge effectively so people actually learn something. A clown teacher is often unaware of how much they are unappreciated and/or disliked.
Mark: "Mr. Gobs failed me again. I pay attention in class but I guess he's not teaching us well enough."
Ryan: "Don't worry about him man. He's a clown teacher. Nobody learns anything from him."