Old politician understood America (7)
I believe the answer is:
tacitus
'old politician' is the definition.
(Roman politician)
'understood america' is the wordplay.
'understood' becomes 'tacit' (I've seen this before).
'america' becomes 'us' (abbreviation for the United States).
'tacit'+'us'='TACITUS'
(Other definitions for tacitus that I've seen before include "Roman politician and historian" , "Writer" , "Old history master" , "Old historian" , "Roman historian, annalist" .)