One’s packed stylish, plain, overnight case (9)
I believe the answer is:
inpatient
'overnight case' is the definition.
(patient or 'case' in hospital overnight)
'one's packed stylish plain' is the wordplay.
'one' becomes 'i' (Roman numeral).
'packed' is an insertion indicator (some letters are packed into others).
'stylish' becomes 'in'.
'plain' becomes 'patent' (synonyms).
'in'+'patent'='inpatent'
'i' placed within 'inpatent' is 'INPATIENT'.
(Other definitions for inpatient that I've seen before include "resident in ward" , "hospital visitor" .)