Not feasible (13)
I believe the answer is:
impracticable
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not feasible</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 impracticable that I've seen before include "Impossible to carry out" , "Unmanageable" , "Not feasible" , "Unable to be done" .)