Row about black river in Rome (5)
I believe the answer is:
tiber
'river in rome' is the definition.
'tiber' can be an answer for 'river' (River Tiber in Italy). I am not sure about the 'in rome' bit.
'row about black' is the wordplay.
'row' becomes 'tier' (a tier is a row of things).
'about' means one lot of letters goes inside another.
'black' becomes 'b' (abbreviation as in pencil classifications).
'tier' placed around 'b' is 'TIBER'.
Can you help me to learn more?
(Other definitions for tiber that I've seen before include "The flower of Rome" , "River of Rome" , "Roman river" , "Italian banker" , "Rome's river" .)