Related Clues:

• Two, for one
• 13
• 2, 3 or 5
• Head anesthesiologist?
• 2 or 3, but not 4
• One providing amusement
• One such as 3, 5, 7, 11, etc
• One with no divisors
• Integer divisible only by itself and by 1
• Excellent anaesthetic for one or two, perhaps
• First-rate painkiller one alone can always divide
• Prepare sleeping tablet? Two or three perhaps
• 2 or 3 — but not 4
• 17- and 38-Across and 11- and 25-Down all start with one
• 3, 5 or 7, but not 9