Showing vile anger when rattled (9)
I believe the answer is:
revealing
'showing' is the definition.
(revealing is a kind of showing)
'vile anger when rattled' is the wordplay.
'when rattled' indicates anagramming the letters (I've seen this in other clues).
'vile'+'anger'='vileanger'
'vileanger' is an anagram of 'REVEALING'.
(Other definitions for revealing that I've seen before include "Giving away" , "Divulging" , "to be telltale" , "Exposing" , "Disclosing" .)