Complain to king on river (6)
I believe the answer is:
grouse
'complain' is the definition.
(grousing is a kind of complaining)
'king on river' is the wordplay.
'king' becomes 'gr'.
'on' means one lot of letters go next to another.
'river' becomes 'ouse' (Yorkshire river**).
'gr'+'ouse'='GROUSE'
'to' is the link.
(Other definitions for grouse that I've seen before include "Gripe; game bird" , "Grumble; game bird" , "1D [GAMEBIRD] < having grumble" , "excellent in New Zealand" , "The complaint of a gamebird" .)