Mum sadly rejected peaceful greeting (6)
I believe the answer is:
salaam
'peaceful greeting' is the definition.
(I've seen this before)
'mum sadly rejected' is the wordplay.
'mum' becomes 'ma' (both can mean mother).
'sadly' becomes 'alas' ('alas' is used to express regret).
'rejected' is a reversal indicator.
'ma'+'alas'='maalas'
'maalas' written backwards gives 'SALAAM'.
(Other definitions for salaam that I've seen before include "Peaceful greeting" , "Muslim greeting signifying peace" , "Masala (anag) - Arabic greeting" , "Oriental greeting with bow of obeisance" , "Muslim greeting gesture" .)