Voluntary (8)
I believe the answer is:
optional
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">voluntary</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for optional that I've seen before include "Open to choice - take your pick" , "something extra?" , "Too plain (anag.)" , "Available as a choice but not obligatory" , "Platoon I join as a matter of choice" .)