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' indicates putting letters inside.
'drink' becomes 'dram' (small alcoholic drink).
'e' put 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" .)