Second simpleton self-satisfied (4)
I believe the answer is:
smug
'self-satisfied' is the definition.
('smug' can be a synonym of 'self-satisfied')
'second simpleton' is the wordplay.
'second' becomes 's' (common abbreviation).
'simpleton' becomes 'mug' (I've seen this before).
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "Smirking" , "Steal" , "Conceited and self-satisfied" , "Like a stuffed shirt" , "Irritatingly pleased with oneself" .)