Lack of conviction (5)
I believe the answer is:
doubt
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">lack of conviction</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)