Sack containing black material (5)
I believe the answer is:
Here is my best explanation:
'material' is the definition.
(fibre is a kind of material)
'sack containing black' is the subsidiary indication.
'sack' becomes 'fire' ('fire' can be a synonym of 'sack'**).
'containing' indicates putting letters inside.
'black' becomes 'b' (abbreviation used in chess).
'fire' going around 'b' is 'fibre'.
(Other definitions for fibre that I've seen before include "Dietary material; thread", "Thread; stamina", "Coarse plant food, low in nutrients", "In brief, a thread", "Brief (anag.)", "Thread, filament", "Filament", "A thread or strand", "(Material composed of) thread(s)" and "Strand?".)