Tail bone (6)
I believe the answer is:
coccyx
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">tail bone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for coccyx that I've seen before include "End of the human spinal column" , "Small triangular bone at base of spine" , "Tailbone" , "Tail bone" , "fused vertebrae" .)