Game bird King George observed on river (6)
I believe the answer is:
grouse
'game bird' is the definition.
(grouse is a kind of game bird)
'king george observed on river' is the wordplay.
'king george' becomes 'gr' (Georgius Rex).
'observed on' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'river' becomes 'ouse' (Yorkshire river).
'gr'+'ouse'='GROUSE'
(Other definitions for grouse that I've seen before include "Complain like game bird" , "Game bird - beef" , "Grumble like game-bird" , "Bellyache" , "Petty complaint" .)