Fiendish setters always take assumed names in crosswords, initially (7)
I believe the answer is:
satanic
'fiendish' is the definition.
('satanic' can be a synonym of 'fiendish')
'setters always take assumed names in crosswords initially' is the wordplay.
'initially' suggests taking the first letters.
The initial letters of 'setters always take assumed names in crosswords' is 'SATANIC'.
(Other definitions for satanic that I've seen before include "wicked" , "Devil-like" , "Nick's" , "like Mephisto" , "Fiendish" .)