Not compulsory (8)
I believe the answer is:
optional
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">not compulsory</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 optional that I've seen before include "Elective, as you choose" , "'Elective, not obligatory (8)'" , "Platoon I join as a matter of choice" , "Open to choice - take your pick" , "Not mandatory" .)