Really vacant on ecstasy and grass (3)
I believe the answer is:
'grass' is the definition.
(I've seen this before)
'really vacant on ecstasy' is the wordplay.
'vacant' says to hollow out the word (remove centre letters) (a vacant or emptied out form of the word).
'on' says to put letters next to each other.
'ecstasy' becomes 'E' (informal term for the drug).
'really' with its centre removed is 'ry'.
'and' acts as a link.
(Other definitions for rye that I've seen before include "Hardy annual cereal grass" , "Type of whisky" , "Drink" , "East Sussex resort" , "Kind of serial or American whiskey" .)