Fail to justify inconclusive opinion (5)
I believe the answer is:
Here is my best explanation:
'fail to justify' is the definition.
'inconclusive opinion' is the subsidiary indication.
'inconclusive' means to remove the last letter.
'opinion' becomes 'belief' (I've seen this before).
'belief' with its final letter taken off is 'belie'.
(Other definitions for belie that I've seen before include "Confute", "Contradict" and "Show to be untrue".)