Foolproof (4-4)
(Other definitions for fail-safe that I've seen before include "It ensures a return to a safety when something goes wrong with a mechanism" , "Designed to prevent accidents in the case of malfunction" , "Unlikely to go wrong" , "Device to ensure proper operation" , "Sure to succeed" .)
'foolproof' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for sure-fire that I've seen before include "Certain of success" , "Certain to succeed - a positive conflagration?" , "Infallible" , "Dead cert" .)
(Another definition for failsafe that I've seen is " designed to prevent crashes".)