Mental sort of dirge (6)
I believe the answer is:
lament
'dirge' is the definition.
(synonyms)
'mental sort' is the wordplay.
'sort' is an anagram indicator.
'mental' anagrammed gives 'LAMENT'.
'of' is the link.
(Other definitions for lament that I've seen before include "Wail" , "Mourn (a person's death)" , "Dirge; bewail" , "Dart-throwing line" , "Bemoan, bewail" .)