Diabolical, as I can't move (7)
I believe the answer is:
satanic
'diabolical' is the definition.
('satanic' can be a synonym of 'diabolical')
'as i can't move' is the wordplay.
'move' is an anagram indicator.
'as'+'i'+'cant'='asicant'
'asicant' with letters rearranged gives 'SATANIC'.
(Other definitions for satanic that I've seen before include "like Mephisto" , "Diabolical" , "Hellish" , "Relating to the devil" , "wicked" .)