Like protective layer originally required to stop widespread disease (9)
I believe the answer is:
epidermic
'like protective layer originally' is the definition.
I can't judge whether this defines the answer.
'required to stop widespread disease' is the wordplay.
'required' becomes 'r' (this might be a standard abbreviation of which I'm unaware).
'to stop' indicates putting letters inside (inserted letters plug or stop a gap).
'widespread disease' becomes 'epidemic' (I've seen this before).
'r' put into '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" .)