Spartacus was one (5)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">spartacus was one</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for slave that I've seen before include "Unpaid worker" , "One bound to serve" , "Person owned for work" , "One in bondage" , "Person in bondage" .)