Complain at king on river (6)
I believe the answer is:
grouse
'complain' is the definition.
(grouse can mean to complain or grumble)
'king on river' is the wordplay.
'king' becomes 'GR' (short for Georgius Rex, King George in Latin).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'river' becomes 'ouse' (Yorkshire river).
'gr'+'ouse'='GROUSE'
'at' acts as a link.
(Other definitions for grouse that I've seen before include "excellent in New Zealand" , "Plump game bird" , "Grumble; game bird" , "Game bird - beef" , "Bellyache" .)