A racket involving kidney-related hormone (9)
I believe the answer is:
adrenalin
'hormone' is the definition.
(adrenalin is a kind of hormone)
'a racket involving kidney-related' is the wordplay.
'racket' becomes 'din' (both can mean a loud noise or disturbance).
'involving' means one lot of letters goes inside another.
'kidney related' becomes 'renal' (I've seen this in another clue).
'din' placed around 'renal' is 'drenalin'.
'a'+'drenalin'='ADRENALIN'
(Other definitions for adrenalin that I've seen before include "Booster" , "A secretion" , "Stimulating hormone" , "that pumps you up" , "Hormone that stimulates the nervous system" .)