Quality of drink that’s uncertain (5)
I believe the answer is:
super
'quality' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?
'drink that's uncertain' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'that's uncertain' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sup'+'er'='SUPER'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for super that I've seen before include "Spiffing!" , "First-rate, top-notch" , "Terrific, ace" , "Prefix meaning over-excellent!" , "Excellent - small-part actor" .)