Be in mourning
I believe the answer is:
grieve
'be in mourning' is the definition.
(I've seen this in another clue)
This is the entire clue.
(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" .)