Self-satisfied son, fool (4)
I believe the answer is:
smug
'self-satisfied' is the definition.
(synonyms)
'son fool' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'fool' becomes 'mug' (synonyms).
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "goody-goody" , "Self-righteous" , "irritating expression" , "Like a stuffed shirt" , "Irritatingly self-satisfied" .)