Drug, Ecstasy, in small case (8)
I believe the answer is:
sedative
'drug' is the definition.
(sedative is a kind of drug)
'ecstasy in small case' is the wordplay.
'ecstasy' becomes 'e' (informal term for the drug).
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'case' becomes 'dative' (dative is a kind of case**).
's'+'dative'='sdative'
'e' going inside 'sdative' is 'SEDATIVE'.
(Other definitions for sedative that I've seen before include "Tranquillising drug" , "Sleep-inducing drug" , "Number" , "Depressant" , "quieter" .)