Unassailable (11)
I believe the answer is:
bulletproof
'unassailable' is the definition.
(I know that unassailable can be written as bulletproof)
This is the entire clue.
(Other definitions for bulletproof that I've seen before include "Able to resist being shot" , "Flawless" , "safe from slugs" , "life-saving protection" , "is warding off flak?" .)