Is unable to (6)
I believe the answer is:
cannot
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">is unable to</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cannot that I've seen before include "Is not in a position to" , "Canton (anag.)" , "Is unabe to" , "Is unable to" , "Is not able to" .)