My friend's used kidney, say (3,4)
I believe the answer is:
old bean
'my friend's' is the definition.
(old-fashioned friendly form of address)
'used kidney say' is the wordplay.
'used' becomes 'old' ('old' can be similar in meaning to 'used').
'kidney say' becomes 'bean' (kidney is a type of bean).
'old'+'bean'='OLD BEAN'
(Other definitions for old bean that I've seen before include "Dated familiar term of address to a man" , "Familiar address to chap" , "Term of affection for a man" , "dear fellow" , "Familiar way to address" .)