Drug-filled idiot's knocked back booze in this (5)
I believe the answer is:
stein
'booze in this' is the definition.
(steins contain beer)
'drug-filled idiot's knocked back' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'filled' is an insertion indicator.
'idiot's' becomes 'nit's' (nit can mean a stupid person).
'knocked back' says the letters should be written in reverse.
'e' going into 'nits' is 'niets'.
'niets' written backwards gives 'STEIN'.
(Other definitions for stein that I've seen before include "Beer-mug" , "Gertrude ---; US writer" , "German tankard" , "German earthenware beer mug" , "Large earthenware beer mug" .)