In the manner of son, sadly (4)
I believe the answer is:
alas
'sadly' is the definition.
('alas' is used to express regret)
'in the manner of son' is the wordplay.
'in the manner of' becomes 'a la' ('a la' means 'in the manner of', as in 'a la mode').
'son' becomes 's' (genealogical abbreviation for son).
'ala'+'s'='ALAS'
(Other definitions for alas that I've seen before include "Too bad, so sad" , "Proper, respectable" , "Expression of grief/pity" , "Expression of grief or misfortune" , "More's the pity" .)