Confident that it's unfailing (7)
I believe the answer is:
certain
I believe this clue is a double definition.
'confident' is the first definition.
(similar in meaning)
'it's unfailing' is the second definition.
(similar in meaning)
'that' is the link.
(Other definitions for certain that I've seen before include "Quite sure, positive" , "Absolutely sure" , "Unquestionable" , "Particular" , "Known beyond doubt" .)