Delete some of the matter as erroneous (5)
I believe the answer is:
erase
'delete' is the definition.
(I know that delete can be written as erase)
'some of the matter as erroneous' is the wordplay.
'some of' indicates a hidden word.
'ERASE' is hidden in the letters of 'the matter as erroneous'.
(Other definitions for erase that I've seen before include "Remove all traces of" , "Efface" , "Remove, wipe out" , "Put out of mind" , "Scratch" .)