Rebel is mailed drug that doctor seizes (9)
I believe the answer is:
dissenter
'rebel' is the definition.
(I've seen this in another clue)
'is mailed drug that doctor seizes' is the wordplay.
'mailed' becomes 'sent' ('send' can be a synonym of 'mail').
'drug' becomes 'e' (E is another name for ecstasy).
'that doctor' becomes 'dr' (abbreviation).
'seizes' indicates putting letters inside.
'is'+'sent'+'e'='issente'
'issente' going within 'dr' is 'DISSENTER'.
(Other definitions for dissenter that I've seen before include "Nonconformist" , "One who disagrees among residents" , "one not C of E" , "A non-orthodox" , "One who disagrees out of tiredness" .)