Drug that's "dropped" (3)
'drug that's dropped' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lsd that I've seen before include "Pounds, shillings and pence of old" , "Old money and hallucinogenic drug" .)
(Other definitions for acid that I've seen before include "Sarcastic (could be sulphuric?)" , "Sharp - tart - kind of house party" , "Corrosive liquid; drug" , "Drug LSD (colloq.)" , "Alkali's opposite" .)