River bank with black content (5)
I believe the answer is:
tiber
'river' is the definition.
(I know that Tiber is a river)
'bank with black content' is the wordplay.
'bank' becomes 'tier' (both can mean a row of similar things).
'with' means one lot of letters goes inside another.
'black' becomes 'sable' (heraldic term for black).
'content' indicates the central letters.
The middle letter of 'sable' is 'b'.
'tier' going around 'b' is 'TIBER'.
(Other definitions for tiber that I've seen before include "divides Rome" , "Italian river with Ostia at its mouth" , "Italian banker" , "River entering the Mediterranean at Ostia" , "Rome's river" .)