Beginning of nephritis can be seen in existing images of the kidney (5)
I believe the answer is:
renal
'of the kidney' is the definition.
(I've seen this before)
'beginning of nephritis can be seen in existing' is the wordplay.
'beginning of' says to take the initial letters.
'can be seen in' is an insertion indicator.
'existing' becomes 'real' (similar in meaning).
The initial letter of 'nephritis' is 'n'.
'n' put within 'real' is 'RENAL'.
'images' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for renal that I've seen before include "around the kidney" , "Pertaining to the kidneys" , "connected with organ" , "Vital organ's" , "Learn (anag) - of organs of the body" .)