Complain to king over climbing plant (6)
I believe the answer is:
kvetch
'complain' is the definition.
(I know that complain can be written as kvetch)
'king over climbing plant' is the wordplay.
'king' becomes 'K' (abbreviation for king in chess).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'climbing plant' becomes 'vetch' (I've seen this in another clue).
'k'+'vetch'='KVETCH'
'to' is the link.
(Other definitions for kvetch that I've seen before include "Complain" , "Whinger" , "grouse" .)