Mouth allowed in vessel (6)
I believe the answer is:
goblet
'vessel' is the definition.
(thesaurus)
'mouth allowed' is the wordplay.
'mouth' becomes 'gob' (informal term for the mouth).
'allowed' becomes 'let' (to let is to allow or permit).
'gob'+'let'='GOBLET'
'in' acts as a link.
(Other definitions for goblet that I've seen before include "Drinking-glass with stem and foot" , "Ornate glass" , "Chalice" , "Large drinking glass with stem and base" , "Large drinking cup" .)