Self-satisfied son, sucker (4)
I believe the answer is:
smug
'self-satisfied' is the definition.
('smug' can be a synonym of 'self-satisfied')
'son sucker' is the wordplay.
'son' becomes 's'.
'sucker' becomes 'mug' ('mug' can be a synonym of 'sucker').
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "Superior" , "Smooth" , "Self-satisfied, complacent" , "Prim" , "irritating expression" .)