Drinking vessel brown, unusually dark (7)
I believe the answer is:
tankard
'drinking vessel' is the definition.
(I know that tankard is a type of drinking vessel)
'brown unusually dark' is the wordplay.
'brown' becomes 'tan' (tan is a kind of brown).
'unusually' indicates an anagram.
'dark' is an anagram of 'kard'.
'tan'+'kard'='TANKARD'
(Other definitions for tankard that I've seen before include "Large drinking vessel" , "something to drink from" , "One-handled drinking mug" , "Drank at (anag)" , "Beer glass" .)