Deride junior ingesting double dose of ecstasy (4)
I believe the answer is:
jeer
'deride' is the definition.
(I've seen this before)
'junior ingesting double dose of ecstasy' is the wordplay.
'junior' becomes 'jr' (abbreviation).
'ingesting' indicates putting letters inside.
'double dose of' shows that letters should be duplicated.
'ecstasy' becomes 'E' (informal term for the drug).
'e' duplicated is 'ee'.
'jr' enclosing 'ee' is 'JEER'.
(Other definitions for jeer that I've seen before include "Mock - laugh at" , "Henry ---, Richard II's enemy, successor" , "Ridicule noisily" , "Gibe" , "Rudely mocking remark" .)