Piece of ivory smuggled in plain case (7)
I believe the answer is:
patient
'case' is the definition.
(a medical case)
'piece of ivory smuggled in plain' is the wordplay.
'piece of' suggests taking the first letters.
'smuggled in' indicates putting letters inside.
'plain' becomes 'patent' (synonyms).
The first letter of 'ivory' is 'i'.
'i' going into 'patent' is 'PATIENT'.
(Other definitions for patient that I've seen before include "One getting medical care" , "Doctor's visitor; composed" , "Doctor's 'customer'" , "Unhurried" , "Sufferer" .)