Shock rattles terribly (7)
I believe the answer is:
startle
'shock' is the definition.
(I know that startle is a type of shock)
'rattles terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'rattles' anagrammed gives 'STARTLE'.
(Other definitions for startle that I've seen before include "Rock" , "Frighten suddenly" , "Rattles (anag.)" , "Take by surprise" , "Take unawares with rattles" .)