Be in session (3)
I believe the answer is:
sit
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be in session</span>' is the definition.<br/ ><span class="explanation-format__explanation">(synonyms)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for sit that I've seen before include "Be seated" , "Use a chair" , "Pose for painter" , "To take the test" , "Brood" .)