Doctor finally probing widespread disease of the skin (9)
I believe the answer is:
epidermic
'of the skin' is the definition.
(I've seen this before)
'doctor finally probing widespread disease' is the wordplay.
'doctor finally' becomes 'r' (I can't explain this - if you can you should believe this answer much more).
'probing' is an insertion indicator (probe can mean to make a hole in).
'widespread disease' becomes 'epidemic' (I've seen this before).
'r' put within 'epidemic' is 'EPIDERMIC'.
Can you help me to learn more?
(Other definitions for epidermic that I've seen before include "affecting our outer layer" , "of outer layer of skin" , "Relating to the skin's outer layer" .)