Hears about hard old stuff that's just been changed slightly (6)
I believe the answer is:
rehash
'old stuff that's just been changed slightly' is the definition.
(things that have been rehashed or reused)
'hears about hard' is the wordplay.
'about' indicates anagramming the letters.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'hears' with letters rearranged gives 'rehas'.
'rehas'+'h'='REHASH'
(Other definitions for rehash that I've seen before include "new disposition" , "Work over (used material)" , "Rework old material" , "That's nothing really new" , "Old material, slightly reworked and used again" .)