One involved in diplomacy meets American historian (7)
I believe the answer is:
tacitus
'historian' is the definition.
(Roman historian)
'one involved in diplomacy meets american' is the wordplay.
'one' becomes 'i' (Roman numeral).
'involved in' is an insertion indicator.
'diplomacy' becomes 'tact' (I've seen this before).
'meets' is a charade indicator (letters next to each other).
'american' becomes 'us'.
'i' inserted inside 'tact' is 'tacit'.
'tacit'+'us'='TACITUS'
(Other definitions for tacitus that I've seen before include "Writer" , "Old history master" , "Roman historian, annalist" , "Old historian" , "Roman politician and historian" .)