Secondary (10)
I believe the answer is:
incidental
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">secondary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(similar in meaning)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for incidental that I've seen before include "form of music" , "Not an essential" , "By the way" , "fortuitous" .)