What's in thoughtless use of old material? (6)
I believe the answer is:
rehash
'use of old material?' is the definition.
(I've seen this before)
'what's in thoughtless' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'in' indicates putting letters inside.
'thoughtless' becomes 'rash' (rash can mean reckless or not thought through).
'eh' placed into 'rash' is 'REHASH'.
(Other definitions for rehash that I've seen before include "new disposition" , "Work over (used material)" , "Old material, slightly reworked and used again" , "Reworking of old material" , "Present again with just the odd change" .)