Warned treadle is unreliable (7)
I believe the answer is:
alerted
'warned' is the definition.
(alerting is a kind of warning)
'treadle is unreliable' is the wordplay.
'is unreliable' indicates an anagram.
'treadle' is an anagram of 'ALERTED'.
(Other definitions for alerted that I've seen before include "Put on guard, warned" , "Given warning" , "Warned, made aware" , "Given signal" , "Tipped off" .)