Criminal trio elected — would have failed this test? (3,8)
I believe the answer is:
lie detector
'would have failed this test?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'criminal trio elected' is the wordplay.
'criminal' is an anagram indicator.
'trio'+'elected'='trioelected'
'trioelected' is an anagram of 'LIE-DETECTOR'.
(Other definitions for lie detector that I've seen before include "Device helping find truth" , "this could indicate a crime" , "Forensic equipment" , "It may check someone's veracity" , "way to discover fiction" .)