Nun ever shaking rattle (7)
I believe the answer is:
unnerve
'rattle' is the definition.
(I know that unnerve is a more specific form of the action rattle)
'nun ever shaking' is the wordplay.
'shaking' indicates an anagram.
'nun'+'ever'='nunever'
'nunever' is an anagram of 'UNNERVE'.
(Other definitions for unnerve that I've seen before include "Frighten" , "Psych out" , "spook" , "Discourage - demoralise" , "Disconcert, make ill at ease" .)