Lack of conviction (5)
I believe the answer is:
doubt
'lack of conviction' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for doubt that I've seen before include "One certainly can't have it" , "Have reservations or be uncertain about" , "Don't be so sure!" , "Uncertainty, mistrust" , "Suspicion" .)