Small shake (6)
I believe the answer is:
tremor
'small shake' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tremor that I've seen before include "make one shake" , "Quiver or vibration" , "A slight shake, body or earth" , "A small shake, perhaps of earth ( 6)" , "Minor earthquake" .)