Something extra good, American (5)
I believe the answer is:
bonus
'something extra' is the definition.
(thesaurus)
'good american' is the wordplay.
'good' becomes 'bon' (I am not sure about this - if you are sure you should believe this answer much more).
'american' becomes 'us' (abbreviation for United States).
'bon'+'us'='BONUS'
Can you help me to learn more?
(Other definitions for bonus that I've seen before include "Tip" , "Premium" , "Good performance reward" , "Extra dividend" , "bribe" .)