Infallible (9)
'infallible' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for foolproof that I've seen before include "incapable of going wrong" , "Not liable to failure" , "Can't be misinterpreted, even by simpleton" , "Not admitting failure" , "sure to work" .)
'infallible' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for unerring that I've seen before include "Always right or accurate" , "Not liable to mistake" , "Pope is said to be such" , "Deadly accurate, always on the mark" , "Dead-eye" .)
'infallible' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sure-fire that I've seen before include "Infallible" , "Dead cert" , "Certain of success" , "Certain to succeed - a positive conflagration?" .)