Greek translation of Tacitus put off American (5)
I believe the answer is:
attic
'greek' is the definition.
(from ancient Athens or Attica)
'translation of tacitus put off american' is the wordplay.
'translation of' is an anagram indicator (translate can mean to transform).
'put off' is a deletion indicator.
'american' becomes 'US' (United States).
'tacitus' with 'us' taken away is 'tacit'.
'tacit' is an anagram of 'ATTIC'.
(Other definitions for attic that I've seen before include "Room at the top of a house" , "28 [GREEK]" , "from Greece" , "Ancient Greek; loft" , "Floor" .)