Be in mourning
I believe the answer is:
grieve
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">be in mourning</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for grieve that I've seen before include "Mourn, feel sorrow" , "Suffer intense sorrow" , "Cause great distress to" , "Feel intense sorrow at someone's death" , "Mourn loss" .)