Debar (8)
I believe the answer is:
preclude
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">debar</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 preclude that I've seen before include "Prevent, prohibit" , "Make impossible" , "Debar" , "'Prevent, forestall (8)'" , "Rule out or eliminate" .)