Russian agreement to conceal material (6)
I believe the answer is:
damask
'material' is the definition.
(I know that damask is a type of material)
'russian agreement to conceal' is the wordplay.
'russian agreement' becomes 'da'.
'to conceal' becomes 'mask' (I've seen this in another clue).
'da'+'mask'='DAMASK'
(Other definitions for damask that I've seen before include "Patterned material" , "reversible material" , "Figured material" , "Table linen material" , "Heavy embroidered linen" .)