Millstone (6)
I believe the answer is:
burden
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">millstone</span>' is the definition.<br/ ><span class="explanation-format__explanation">(thesaurus)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for burden that I've seen before include "A heavy load or difficulty" , "Unwelcome load" , "Encumbrance" , "Onus or load" , "Oppressive duty" .)