Drowsiness (10)
I believe the answer is:
somnolence
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">drowsiness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that drowsiness can be written as somnolence)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for somnolence that I've seen before include "in their sleep" , "Drowsiness" , "Sleepiness" , "Dropping off" .)