Unnerves (6)
(Other definitions for alarms that I've seen before include "Warning clocks that wake us up" , "Scares or startles" , "Warnings of danger" , "Warning signals" , "Frightens, makes afraid" .)
(Other definitions for jars that I've seen before include "Grates" , "Wide-mouthed containers" , "Jam pots" , "Vessels with a wide mouths" .)
'unnerves' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for rattles that I've seen before include "Noisy toys for babies" , "Starlet (anag)" .)