Essentially American hero (7)
I believe the answer is:
perseus
'hero' is the definition.
(I've seen this before)
'essentially american' is the wordplay.
'essentially' becomes 'per se' (I've seen this in another clue).
'american' becomes 'us' (abbreviation for United States).
'perse'+'us'='PERSEUS'
(Other definitions for perseus that I've seen before include "Medusan killer" , "Gorgon-slayer and rescuer of Andromeda" , "Medusa's slayer" , "'Having slain Medusa, he became a constellation (7)'" , "He beheaded Medusa" .)