Clue: Doctor drinks the greatest, most superior cocktail (6)
Answer: MALIBU
Explanation: Doctor drinks the greatest, most superior cocktail (6)
**MB** (Bachelor of Medicine, so "doctor") *drinks* (Mohammad) **ALI** ("The Greatest") + **U** ("most superior") 
*I'm a teetotaller, but I'm pretty sure Malibu is a coconut-flavoured rum that is an ingredient in many cocktails, but is not in itself a cocktail.