Roman historian understood America (7)
I believe the answer is:
tacitus
'roman historian' is the definition.
(I know that Tacitus is a historian)
'understood america' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'america' becomes 'US' (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" , "Old historian" , "Writer" .)