Solitary, lone (6)
I believe the answer is:
single
<span class="explanation-format__ClueExplanation">I believe this is a double definition.<br/ ><br/ >'<span class="explanation-format__original">solitary</span>' is the first definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >'<span class="explanation-format__original">lone</span>' is the second definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ ></span>
(Other definitions for single that I've seen before include "Unaccompanied" , "Free" , "Unique" , "One-way ticket" , "Not married, just the one" .)