Crossword Genius

Son, idiot, complacent (4)

Ross

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" .)

I've seen this clue in The Sun.
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