Twin birds following river (5)
I believe the answer is:
remus
'twin' is the definition.
(legendary twins Romulus and Remus)
'birds following river' is the wordplay.
'birds' becomes 'emus' (emu is a kind of bird).
'following' is a charade indicator (letters next to each other).
'river' becomes 'r' (abbreviation).
'emus' put after 'r' is 'REMUS'.
(Other definitions for remus that I've seen before include "Uncle character in US folk stories" , "Twin founder of Rome" , "Twin of Romulus" , "partner in City foundation" , "US relative who did accounts" .)