Understood by American historian (7)
I believe the answer is:
tacitus
'historian' is the definition.
(Roman historian)
'understood by american' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'by' says to put letters next to each other.
'american' becomes 'us' (abbreviation for United States).
'tacit'+'us'='TACITUS'
(Other definitions for tacitus that I've seen before include "Roman historian, annalist" , "Old history master" , "Roman politician and historian" , "Writer" , "Old historian" .)