Sign inexperienced journalist's written up coarse material (7)
I believe the answer is:
buckram
'coarse material' is the definition.
(I've seen this before)
'sign inexperienced journalist's written up' is the wordplay.
'sign' becomes 'mark' (mark is a kind of sign).
'inexperienced journalist' becomes 'cub' (a cub reporter is an inexperienced journalist).
'written up' says the letters should be written backwards.
'mark'+'cub'='markcub'
'markcub' in reverse letter order is 'BUCKRAM'.
(Other definitions for buckram that I've seen before include "Rough fabric sized to be stiff" , "Coarse stuff" , "Bookbinding material - cub mark (anag)" , "Cloth stiffened with size" , "stiffening" .)