Historian understood country (7)
I believe the answer is:
tacitus
'historian' is the definition.
(Roman historian)
'understood country' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'country' becomes 'us' (U.S. is an example).
'tacit'+'us'='TACITUS'
(Other definitions for tacitus that I've seen before include "Roman politician and historian" , "Writer" , "Old history master" , "Roman historian, annalist" , "Old historian" .)