“Please explain what a” one-time rosh yeshiva” is?”
Wasnt directed to me. But I hope I can help
“One-time” = Former
“rosh yeshiva” = Head of the academy
Thus ” one-time rosh yeshiva” = His former Head of Academy
(Could also mean The head of his former academy or the Former head of his academy was that your source of confusion? The former is the most likely