Frighten by shaking rattles (7)
I believe the answer is:
startle
'frighten' is the definition.
(I've seen this in another clue)
'shaking rattles' is the wordplay.
'shaking' indicates an anagram.
'rattles' with letters rearranged gives 'STARTLE'.
'by' acts as a link.
(Other definitions for startle that I've seen before include "Scare" , "Surprise greatly, alarm" , "Cause to feel sudden alarm" , "Astonish" , "Shock (someone)" .)