I’d touch of nephritis in vital organ (6)
I believe the answer is:
kidney
'organ' is the definition.
(kidney is a kind of organ)
'i'd touch of nephritis in vital' is the wordplay.
'touch of' suggests taking the first letters.
'in' indicates putting letters inside.
'vital' becomes 'key' (key can mean important).
The initial letter of 'nephritis' is 'n'.
'id'+'n'='idn'
'idn' inserted inside 'key' is 'KIDNEY'.
(Other definitions for kidney that I've seen before include "Bean-shaped organ" , "body part" , "class" , "Excretory organ" , "Glandular organ that removes waste matter from blood" .)