Sat, dizzy with spasm, motionless (6)
I believe the answer is:
static
'motionless' is the definition.
('static' can be a synonym of 'motionless')
'sat dizzy with spasm' is the wordplay.
'dizzy' indicates an anagram.
'with' is a charade indicator (letters next to each other).
'spasm' becomes 'tic' (tic is a kind of spasm).
'sat' with letters rearranged gives 'sta'.
'sta'+'tic'='STATIC'
(Other definitions for static that I've seen before include "Inactive or inert" , "Not active or moving" , "Not active, immobile" , "Unmoving, unchanging" , "Atmospheric disturbance" .)