I, in river, go belly up (4)
I believe the answer is:
fail
'go belly up' is the definition.
(I've seen this in another clue)
'i in river' is the wordplay.
'in' means one lot of letters goes inside another.
'river' becomes 'fal' (Cornish river).
'i' placed inside 'fal' is 'FAIL'.
(Other definitions for fail that I've seen before include "Be unsuccessful, say in exam" , "collapse" , "Flunk" , "Come to naught" , "Go bankrupt" .)