Grieve over (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
mourn
'grieve over' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mourn that I've seen before include "Weep for" , "Show sorrow at death" , "'Miss'" , "Express sadness" , "Regret (a loss)" .)