Prosecutor not to reveal being a silk (6)
I believe the answer is:
damask
'a silk' is the definition.
(type of silk)
'prosecutor not to reveal' is the wordplay.
'prosecutor' becomes 'D.A.' (District Attorney).
'not to reveal' becomes 'mask' (to mask something is to conceal it).
'da'+'mask'='DAMASK'
'being' is the link.
(Other definitions for damask that I've seen before include "Silk or linen material woven with pattern on either side" , "Lustrous cloth" , "Patterned cloth" , "heavy embossed table linen" , "Cloth, originally Syrian" .)