Drunk certain to be missing one drink (6)
I believe the answer is:
nectar
'drink' is the definition.
(nectar is a kind of drink)
'drunk certain to be missing one' is the wordplay.
'drunk' indicates anagramming the letters.
'to be missing' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'certain' with 'i' taken out is 'certan'.
'certan' is an anagram of 'NECTAR'.
(Other definitions for nectar that I've seen before include "That's sweet" , "sweet thing" , "Recant (anag.)" , "Perfect drink for trance" , "Beverage of the gods" .)