Off one's head, swallowing drug or alcoholic drink (4)
I believe the answer is:
mead
'alcoholic drink' is the definition.
(I know that mead is a type of alcoholic drink)
'off one's head swallowing drug' is the wordplay.
'off one's head' becomes 'mad' (off one's head can informally mean mad).
'swallowing' is an insertion indicator.
'drug' becomes 'e' (E is another name for ecstasy).
'mad' placed around 'e' is 'MEAD'.
'or' acts as a link.
(Other definitions for mead that I've seen before include "Honey-based alcoholic drink" , "Honey drink; lea" , "US anthropologist, Margaret d. 1978" , "Honeyed drink; a lea" , "sweet drink" .)