Alarm by shaking rattles (7)
I believe the answer is:
startle
'alarm' is the definition.
(I've seen this before)
'shaking rattles' is the wordplay.
'shaking' indicates anagramming the letters.
'rattles' with letters rearranged gives 'STARTLE'.
'by' acts as a link.
(Other definitions for startle that I've seen before include "Make jump" , "Scare" , "Astonish" , "Rock" , "Take (someone) aback" .)