Unhappy with river in flood (5)
I believe the answer is:
drown
'flood' is the definition.
(I've seen this before)
'unhappy with river' is the wordplay.
'unhappy' becomes 'down' (down can mean sad).
'with' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'down' going around 'r' is 'DROWN'.
'in' acts as a link.
(Other definitions for drown that I've seen before include "Become submerged" , "Perish in water" , "Kill by submersion" , "Completely conceal" , "Flood; kill" .)