Item originally found in plain case (7)
I believe the answer is:
patient
'case' is the definition.
(a medical case)
'item originally found in plain' is the wordplay.
'originally' suggests taking the first letters.
'found in' indicates putting letters inside.
'plain' becomes 'patent' (synonyms).
The first letter of 'item' is 'i'.
'i' placed inside 'patent' is 'PATIENT'.
(Other definitions for patient that I've seen before include "Doctor's visitor; composed" , "Uncomplaining; invalid" , "sick person" , "one receiving medical treatment" , "case for treatment" .)