He's annoyingly smug but will later go nuts (7)

'he's annoyingly smug' is the definition.
The definition suggests a singular noun which matches the answer.

'will later go nuts' is the wordplay.
'will' says to put letters next to each other (I've seen this in other clues).
'nuts' is an anagram indicator (nuts can mean crazy).
'later' put next to 'go' is 'latergo'.
'latergo' is an anagram of 'GLOATER'.

'but' is the link.

Can you help me to learn more?