Receiver has thrown kit into a river (9)
I believe the answer is:
addressee
'receiver' is the definition.
(addressee is a kind of receiver)
'thrown kit into a river' is the wordplay.
'thrown kit' becomes 'dress' (I am not sure about this - if you are sure you should believe this answer much more).
'into' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'a'+'dee'='adee'
'dress' put into 'adee' is 'ADDRESSEE'.
'has' acts as a link.
Can you help me to learn more?
(Other definitions for addressee that I've seen before include "One contacted" , "The one to whom the message is sent" , "One sent letter" , "Intended recipient of letter" , "one who's likely to get the post" .)