Causing panic in the manner of king on film (8)
I believe the answer is:
alarmist
'causing panic' is the definition.
(I've seen this before)
'in the manner of king on film' is the wordplay.
'in the manner of' becomes 'ala' (I've seen this before).
'king' becomes 'r' (r is abbreviation for rex).
'on' means one lot of letters go next to another.
'film' becomes 'mist' (I've seen this in other clues**).
'ala'+'r'+'mist'='ALARMIST'
(Other definitions for alarmist that I've seen before include "Creating needless worry" , "Calamity Jane" , "unduly frightening" , "Scaremonger - the boy who cried wolf was one" , "One crying wolf" .)