Mental sort of song (6)
I believe the answer is:
lament
'song' is the definition.
(I know that lament is a type of song)
'mental sort' is the wordplay.
'sort' indicates an anagram.
'mental' is an anagram of 'LAMENT'.
'of' is the link.
(Other definitions for lament that I've seen before include "Cry of sorrow" , "Bemoan, bewail" , "Mental (anag)" , "Mourn , bewail the loss of" , "requiem" .)