Tendon, ligament (5)
I believe the answer is:
sinew
<span class="explanation-format__ClueExplanation">I believe this clue is a double definition.<br/ ><br/ >'<span class="explanation-format__original">tendon</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >'<span class="explanation-format__original">ligament</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ ></span>
(Other definitions for sinew that I've seen before include "power source" , "Force" , "Tendon - ligament" , "source of physical strength" , "It indicates strength" .)