Mourn, express sorrow (6)
I believe the answer is:
lament
This clue is a double definition.
'mourn' is the first definition.
(I've seen this before)
'express sorrow' is the second definition.
(I've seen this before)
(Other definitions for lament that I've seen before include "Dirge; bewail" , "Mourn in mental way (6)" , "requiem" , "Show of sorrow" , "Expression of sorrow" .)