Drinking-vessel’s small mouth? (6)
I believe the answer is:
goblet
This clue is a double definition.
'drinking-vessel's' is the first definition.
(a goblet is a drinking vessel)
'small mouth?' is the second definition.
(goblet could whimsically mean a small gob or mouth)
(Other definitions for goblet that I've seen before include "Large handleless drinking-cup" , "Large drinking glass with stem and base" , "Drinking glass with stem and foot" , "Stemmed drinking glass" , "Drinking-glass with stem and foot" .)