River delta round top of Nile (3)
I believe the answer is:
don
'river' is the definition.
(Don is an example)
'delta round top of nile' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'round' becomes 'o' (round can mean a round object).
'top of' suggests taking the first letters.
The initial letter of 'nile' is 'n'.
'd'+'o'+'n'='DON'
(Other definitions for don that I've seen before include "Mafia boss" , "Professor" , "Put on; lecturer" , "Put on for the Mafia chief and Spanish aristocrat" , "A mafia chief or Spanish grandee" .)