Sack with internal black thread (5)
I believe the answer is:
fibre
'thread' is the definition.
(I've seen this in another clue)
'sack with internal black' is the wordplay.
'sack' becomes 'fire' (both can mean to dismiss from a job).
'with internal' indicates putting letters inside.
'black' becomes 'b' (abbreviation as in pencil classifications).
'fire' going around 'b' is 'FIBRE'.
(Other definitions for fibre that I've seen before include "(Material composed of) thread(s)" , "Coarse plant food, low in nutrients" , "In brief, a thread" , "Brief (anag.)" , "Textile thread; roughage" .)