Complain to old king of river (6)
I believe the answer is:
grouse
'complain' is the definition.
(grouse can mean to complain or grumble)
'old king of river' is the wordplay.
'old king' becomes 'GR' (Georgius Rex, Latin for King George).
'of river' becomes 'ouse' (I can't justify this - if you can you should believe this answer much more).
'gr'+'ouse'='GROUSE'
'to' is the link.
Can you help me to learn more?
(Other definitions for grouse that I've seen before include "1D [GAMEBIRD] < having grumble" , "Gripe; game bird" , "Plump game bird" , "Complain pettily" , "Grumble; game bird" .)