Extreme radical (7)
'extreme radical' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for jacobin that I've seen before include "French revolutionary" , "What can make one a wild man" , "French radical" , "pigeon" , "Extreme radical" .)
This clue is a double definition.
'extreme' is the first definition.
(I've seen this in another clue)
'radical' is the second definition.
(drastic can mean radical or extreme)
(Other definitions for drastic that I've seen before include "'Dramatic, extreme (7)'" , "Violent Doctor Crippen starts drinking wine" , "Grave" , "Severe and far-reaching" , "Radical, far-reaching" .)