Rosy stuff? Half of that’s black stuff (5)
I believe the answer is:
attar
'rosy stuff?' is the definition.
(oil typically made from rose petals)
'half of that's black stuff' is the wordplay.
'half of' means to take half the letters.
'black stuff' becomes 'tar' (tar is a black substance).
'that' halved is 'at'.
'at'+'tar'='ATTAR'
(Other definitions for attar that I've seen before include "Fragrant oil from flowers" , "Oil from flowers for perfume" , "Fragrant essence usually associated with the damask rose" , "Essential oil from flowers" , "Fragrant oil obtained from roses" .)