"Corruption, the most ____ symptom of constitutional liberty” (Edward Gibbon in Decline and Fall of the Roman Empire) (10)

I believe the answer is:

infallible

(Other definitions for infallible that I've seen before include "Always right" , "Unable to make a mistake" , "Incapable of error" , "faultless" , "Always working" .)

I've seen this clue in The Times.

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