Triangular pelvic bones (5)
I believe the answer is:
sacra
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">triangular pelvic bones</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sacra that I've seen before include "Bones that form the human pelvic arch" , "Triangular bones" .)