Shudder (6)
I believe the answer is:
tremor
'shudder' is the definition.
(I know that shudder can be written as tremor)
This is the entire clue.
(Other definitions for tremor that I've seen before include "Slight earthquake" , "make one shake" , "feeling of excitement" , "A slight shake, perhaps of earth" , "Quivering" .)