City on the Tiber (4)
I believe the answer is:
rome
'city on the tiber' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rome that I've seen before include "Colosseum city" , "When here, do as the citizens do" , "Italian capital (where one Ac died)" , "Capital of Italy" , "All roads lead to it" .)