Teacher for just a day, informally

SUB

Think of the short-term stand-in who might take over when your regular instructor is out—often found in schools or offices.

This clue plays on the informal term for a substitute teacher—someone who fills in for the regular teacher, often just for a single day. "Teacher for just a day" hints at the temporary role, while "informally" signals the answer is a casual or shortened word. "Sub" fits perfectly as both a common abbreviation for "substitute" and a quick, everyday way to refer to the job.