Complacent, son, gullible person (4)
I believe the answer is:
smug
'complacent' is the definition.
(smug can mean complacent or self-satisfied)
'son gullible person' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'gullible person' becomes 'mug' (mug can mean a gullible or foolish person).
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "Superior" , "Arrogantly complacent" , "Self-satisfied, complacent" , "Be envious of" , "Steal" .)