Inability to sleep (8)
I believe the answer is:
insomnia
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">inability to sleep</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 insomnia that I've seen before include "Condition affecting sleep" , "In restless state" , "Sleep disorder" , "Inability to drop off" , "Habitual inability to sleep (8)" .)