Dark beer, pint, drunk with meal (12)
I believe the answer is:
impenetrable
'dark' is the definition.
(thesaurus)
'beer pint drunk with meal' is the wordplay.
'drunk with' indicates an anagram.
'beer'+'pint'='beerpint'
'beerpint' mixed up with 'meal' makes 'IMPENETRABLE'.
(Other definitions for impenetrable that I've seen before include "Impossible to enter" , "Unintelligible" , "Impossible to understand" , "Abstruse" , "Enigmatic" .)