Son, idiot, complacent (4)
I believe the answer is:
smug
'complacent' is the definition.
(smug can mean complacent or self-satisfied)
'son idiot' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'idiot' becomes 'mug' (a gullible or foolish person).
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "Self-satisfied about gums" , "Overly complacent" , "Like a stuffed shirt" , "Arrogantly self-satisfied" , "Self-righteous" .)