Leaderless fanatics terribly devilish (7)
I believe the answer is:
satanic
'devilish' is the definition.
(I've seen this before)
'leaderless fanatics terribly' is the wordplay.
'leaderless' means to remove the first letter.
'terribly' indicates an anagram.
'fanatics' with its initial letter taken off is 'anatics'.
'anatics' with letters rearranged gives 'SATANIC'.
(Other definitions for satanic that I've seen before include "like Mephisto" , "Bad" , "Devil-like" , "wicked" , "Diabolical" .)