Full assembly (6)
I believe the answer is:
plenum
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">full assembly</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for plenum that I've seen before include "completely filled space" , "Meeting of whole body" , "group meeting" , "Full assembly" , "Fully attended meeting" .)