Non-mandatory (8)
I believe the answer is:
optional
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">non-mandatory</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I know that non-mandatory can be written as optional)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for optional that I've seen before include "Not compulsory, discretionary" , "Too plain (anag.)" , "'Left to choice, not compulsory (8)'" , "Such as one can take or leave" , "'Elective, not obligatory (8)'" .)