Fancy drug slipped into drink (5)
I believe the answer is:
dream
'fancy' is the definition.
(both can mean a fantasy)
'drug slipped into drink' is the wordplay.
'drug' becomes 'e' (E is another name for ecstasy).
'slipped into' is an insertion indicator.
'drink' becomes 'dram' (small alcoholic drink).
'e' going inside 'dram' is 'DREAM'.
(Other definitions for dream that I've seen before include "Cherished hope; reverie" , "Fantasy; imagine" , "Fantasise, imagine" , "Switch off" , "Part of sleep" .)