Rigorous; severe (6)
I believe the answer is:
strict
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">rigorous</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that rigorous can be written as strict)</span><br/ ><br/ >'<span class="explanation-format__original">severe</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ ></span>
(Other definitions for strict that I've seen before include "Not allowing laxity" , "Uncompromising in discipline" , "Allowing no deviation from a standard" , "Unbending, precise" , "Exact, rigorous" .)