Sack containing black material (5)
I believe the answer is:
fibre
'material' is the definition.
(fibre is a kind of material)
'sack containing black' is the wordplay.
'sack' becomes 'fire' ('fire' can be a synonym of 'sack'**).
'containing' means one lot of letters goes inside another.
'black' becomes 'b' (as in pencil classifications).
'fire' enclosing 'b' is 'FIBRE'.
(Other definitions for fibre that I've seen before include "Coarse plant food, low in nutrients" , "(Material composed of) thread(s)" , "Threadlike substance" , "Thread, strand" , "Brief (anag.)" .)