One accepted new complaint (4)
I believe the answer is:
acne
'complaint' is the definition.
(acne is a kind of disease)
'one accepted new' is the wordplay.
'one' becomes 'ace' ('ace' can be a synonym of 'one'**).
'accepted' is an insertion indicator.
'new' becomes 'n'.
'ace' enclosing 'n' is 'ACNE'.
(Other definitions for acne that I've seen before include "Skin affliction" , "Skin rash" , "Adolescent spots" , "Problem to face in" , "Face problems" .)