Personal data not originally kept by important organ (6)
I believe the answer is:
kidney
'organ' is the definition.
(I know that kidney is a type of urinary organ)
'personal data not originally kept by important' is the wordplay.
'personal data' becomes 'ID' (identification).
'originally' indicates taking the first letters.
'kept by' is an insertion indicator.
'important' becomes 'key' (similar in meaning).
The initial letter of 'not' is 'n'.
'id'+'n'='idn'
'idn' going into 'key' is 'KIDNEY'.
(Other definitions for kidney that I've seen before include "Bean-shaped organ" , "body part" , "Organ at back of abdomen" , "Sort" , "Organ associated with steak" .)