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