Bellybutton buildup (4)
I believe the answer is:
lint
'bellybutton buildup' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lint that I've seen before include "Napped cloth used to dress wounds" , "Fabric for dressing wounds" , "Soft material for dressing wounds" , "Wound dressing" , "Wound-dressing fabric" .)