Row across British or Italian river (5)
I believe the answer is:
tiber
'italian river' is the definition.
(I've seen this before)
'row across british' is the wordplay.
'row' becomes 'tier' (tier is a kind of row).
'across' is an insertion indicator.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'tier' going around 'b' is 'TIBER'.
'or' acts as a link.
(Other definitions for tiber that I've seen before include "River of Rome" , "Rome's river" , "The flower of Rome" , "divides Rome" , "River entering the Mediterranean at Ostia" .)