Locum

There was a certain amount of feeling evidenced against Dr. Quentin. To begin with, he was not the regular doctor, only a locum, putting in a month’s work, whilst Dr. Bolitho was away on a well-earned holiday.

Source: wiktionary

"I asked about his Joes, he said the Joes could look after themselves and if Jack was bothered about them he could send a locum."

Source: wiktionary

-- "I suppose you wouldn't like to do a locum for a month on the South coast? Three guineas a week with board and lodging." -- "I wouldn't mind," said Philip. -- "It's at Farnley, in Dorsetshire. Doctor South. You'd have to go down at once; his assistant has developed mumps. I believe it's a very pleasant place."

Source: wiktionary