Complacent (4)
'complacent' is the definition.
(smug can mean complacent or self-satisfied)
This is the entire clue.
(Other definitions for smug that I've seen before include "Smirking" , "Irritatingly pleased with oneself" , "Unpleasantly complacent" , "Self-satisfied about gums" , "Self-satisfied, complacent" .)
(Other definitions for cocky that I've seen before include "Full of oneself" , "Boldly conceited" , "Made observation" , "Impudently self-assured" , "Remove errors from (program)" .)