Brown, curiously dark drinking vessel (7)
I believe the answer is:
tankard
'drinking vessel' is the definition.
(tankard is a kind of drinking vessel)
'brown curiously dark' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'curiously' indicates an anagram.
'dark' is an anagram of 'kard'.
'tan'+'kard'='TANKARD'
(Other definitions for tankard that I've seen before include "something to drink from" , "Metal beer mug" , "Beer-mug" , "One-handled drinking mug" , "One-handled beer mug" .)