A greeting is regrettably returned before morning (6)
I believe the answer is:
salaam
'a greeting' is the definition.
(I've seen this before)
'regrettably returned before morning' is the wordplay.
'regrettably' becomes 'alas' ('alas' is said before regrettable news).
'returned' says the letters should be written in reverse.
'before' says to put letters next to each other.
'morning' becomes 'am'.
'alas' written backwards gives 'sala'.
'sala'+'am'='SALAAM'
'is' acts as a link.
(Other definitions for salaam that I've seen before include "Oriental salutation meaning peace" , "Oriental greeting like low bow" , "An Oriental salute or bow" , "Gesture made by foreigner" , "Oriental greeting or homage" .)