Strain or tension (6)
I believe the answer is:
stress
<span class="explanation-format__ClueExplanation">This clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">strain</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(I know that strain can be written as stress)</span><br/ ><br/ >'<span class="explanation-format__original">tension</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(I know that tension can be written as stress)</span><br/ ><br/ ></span>
(Other definitions for stress that I've seen before include "Force" , "State mental or emotional strain" , "Put weight on" , "Emphasize" , "Pressure or strain" .)