Prosecutor with impassive face, a silk (6)
I believe the answer is:
damask
'a silk' is the definition.
(I've seen this before)
'prosecutor with impassive face' is the wordplay.
'prosecutor' becomes 'da' (D.A. is an example).
'with' is a charade indicator (letters next to each other).
'impassive face' becomes 'mask'.
'da'+'mask'='DAMASK'
(Other definitions for damask that I've seen before include "Material, originally Syrian" , "Figured material" , "Red" , "A silk fabric" , "heavy embossed table linen" .)