Mental arrangement for a song (6)
I believe the answer is:
lament
'song' is the definition.
(lament is a kind of song)
'mental arrangement' is the wordplay.
'arrangement' is an anagram indicator.
'mental' is an anagram of 'LAMENT'.
'for a' is the link.
(Other definitions for lament that I've seen before include "Bemoan, bewail" , "Dirge; bewail" , "Mourn (a person's death)" , "Mourn in mental way (6)" , "complaint" .)