One with plain dressing case (7)
I believe the answer is:
patient
'case' is the definition.
(a medical case)
'one with plain dressing' is the wordplay.
'one with' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'plain' becomes 'patent' (synonyms).
'dressing' means one lot of letters goes inside another (inserted letters are dressed or clad in others).
'i' placed inside 'patent' is 'PATIENT'.
Can you help me to learn more?
(Other definitions for patient that I've seen before include "Cool" , "Sufferer" , "Calm" , "one of interest to medics?" , "Forgiving" .)