Demanding, challenging (8)
I believe the answer is:
exacting
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">demanding</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(exacting is a kind of demanding)</span><br/ ><br/ >'<span class="explanation-format__original">challenging</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ ></span>
(Other definitions for exacting that I've seen before include "Making great demands like difficult task" , "Unreasonable in making demands" , "Making rigorous demands" , "Requiring hard work and attention" , "Harsh" .)