Look drunk, becoming impenetrable (8)
I believe the answer is:
airtight
'impenetrable' is the definition.
(I've seen this before)
'look drunk' is the wordplay.
'look' becomes 'air' (I've seen this before).
'drunk' becomes 'tight' (I've seen this in other clues).
'air'+'tight'='AIRTIGHT'
'becoming' is the link.
(Other definitions for airtight that I've seen before include "(Of a defence) having no weaknesses" , "Not allowing gas to enter or escape" , "Sure" , "Firmly closed" , "Not allowing gas to pass" .)