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" .)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">foolproof</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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".)