Lawless chief gripped by terror after beheading (8)
I believe the answer is:
anarchic
'lawless' is the definition.
(synonyms)
'chief gripped by terror after beheading' is the wordplay.
'chief' becomes 'arch' (synonyms).
'gripped by' is an insertion indicator.
'terror' becomes 'panic' (synonyms).
'after beheading' means to remove the first letter.
'panic' with its initial letter taken away is 'anic'.
'arch' inserted within 'anic' is 'ANARCHIC'.
(Other definitions for anarchic that I've seen before include "lacking proper rule" , "Car chain (anag.)" , "Uncontrolled or lawless" , "Lawless, lacking government or control" , "Following no rules" .)