Stop the flow of waste material (4)
I believe the answer is:
stem
'stop the flow' is the definition.
(I've seen this before)
'of waste material' is the wordplay.
'of' indicates the answer is hidden within the clue.
'STEM' is hidden in the letters of 'waste material'.
(Other definitions for stem that I've seen before include "supporter of flag, for example" , "Slender vertical part, glass or plant" , "Halt flow" , "Stalk; restrict (a flow)" , "Block" .)