Vessel is a sort of dark tan (7)
I believe the answer is:
tankard
'vessel' is the definition.
(I know that tankard is a type of drinking vessel)
'sort of dark tan' is the wordplay.
'sort of' indicates an anagram.
'dark'+'tan'='darktan'
'darktan' anagrammed gives 'TANKARD'.
'is a' acts as a link.
(Other definitions for tankard that I've seen before include "Large drinking vessel" , "Tall beer mug" , "Metal beer mug" , "Beer-glass" , "One-handled drinking mug" .)