Militant leader in Exeter excited radical (7)
I believe the answer is:
extreme
'radical' is the definition.
(thesaurus)
'militant leader in exeter excited' is the wordplay.
'leader' says to take the initial letters.
'in' indicates putting letters inside.
'excited' indicates anagramming the letters.
The initial letter of 'militant' is 'm'.
'exeter' is an anagram of 'extree'.
'm' placed inside 'extree' is 'EXTREME'.
(Other definitions for extreme that I've seen before include "Far from centre" , "Furthest or highest degree of something" , "Immoderate" , "Unrestrained" , "Farthest - to the highest degree" .)