Hours of darkness (5)
I believe the answer is:
night
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">hours of darkness</span>' is the definition.<br/ ><span class="explanation-format__explanation">(dark time of day)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for night that I've seen before include "Dark period" , "Hours of darkness" , "That's when it's darkest" , "- School, - owl" , "Period of darkness" .)