Clue: Wherein one gets stoned — awful dump, I gathered (5,3)
Answer: OPIUM DEN
Explanation: Definition: Wherein one gets stoned
The definition also forms part of the wordplay (or, with a little licence, the clue may be regarded as an &lit, which is probably the intention). An envelope ('wherein … gathered') of PIUMD, an anagram (awful') of 'dump I' in OEN, an anagram ('gets stoned') of 'one'.