Protects a shaky witness (9)
I believe the answer is:
spectator
'witness' is the definition.
('spectator' can be a synonym of 'witness')
'protects a shaky' is the wordplay.
'shaky' indicates anagramming the letters.
'protects'+'a'='protectsa'
'protectsa' with letters rearranged gives 'SPECTATOR'.
(Other definitions for spectator that I've seen before include "Person watching" , "He attends matches" , "Onlooker at game" , "weekly publication" , "Witness" .)