Impenetrable song offered by drunk (8)
I believe the answer is:
airtight
'impenetrable' is the definition.
(I've seen this before)
'song offered by drunk' is the wordplay.
'song' becomes 'air' (air can mean a piece of music).
'offered by' means one lot of letters go next to another.
'drunk' becomes 'tight' (slang term).
'air'+'tight'='AIRTIGHT'
(Other definitions for airtight that I've seen before include "beyond question" , "With no opening" , "Sure" , "not letting out a breath?" , "Firmly closed" .)