Mattress filling (5)
I believe the answer is:
kapok
'mattress filling' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for kapok that I've seen before include "Stuffing material" , "A plant fibre used for stuffing and insulation" , "Silk-cotton" , "filler" , "Natural stuffing fibre" .)