Son, fool, too confident (4)
I believe the answer is:
smug
'too confident' is the definition.
(I've seen this in another clue)
'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 "Be envious of" , "Conceited and self-satisfied" , "looking pleased with itself" , "Self-satisfied, complacent" , "goody-goody" .)