Energy in a small drink? That's fantasy (5)
I believe the answer is:
dream
'fantasy' is the definition.
(dream is a kind of fantasy)
'energy in a small drink?' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'in' is an insertion indicator.
'a small drink?' becomes 'dram' (I've seen this before).
'e' going inside 'dram' is 'DREAM'.
'that's' acts as a link.
(Other definitions for dream that I've seen before include "Fantasise, imagine" , "Fantasy; ideal" , "Bedroom vision?" , "Nocturnal vision" , "Reverie or nightmare" .)