River of soup – no hoax! (5)
I believe the answer is:
somme
'river' is the definition.
(I've seen this before)
'soup no hoax' is the wordplay.
'soup' becomes 'consomme' (consomme is a kind of soup).
'no' indicates named letters should be taken away.
'hoax' becomes 'con'.
'consomme' with 'con' taken out is 'SOMME'.
'of' is the link.
(Other definitions for somme that I've seen before include "Battle site in France" , "WWI battle site" , "World War I battlefield around French river" , "French 2 down" , "River of northern France" .)