A greeting is sadly returned before morning (6)
I believe the answer is:
salaam
'a greeting' is the definition.
(salaam is an Arabic greeting)
'sadly returned before morning' is the wordplay.
'sadly' becomes 'alas' ('alas' is used to express regret).
'returned' says the letters should be written backwards.
'before' means one lot of letters go next to another.
'morning' becomes 'am'.
'alas' backwards is 'sala'.
'sala'+'am'='SALAAM'
'is' is the link.
(Other definitions for salaam that I've seen before include "Oriental greeting with bow of obeisance" , "Peaceful greeting" , "Gesture made by foreigner" , "Arabic salutation" , "Muslim greeting gesture" .)