Tarnish one vessel containing unknown almond paste (8)
I believe the answer is:
marzipan
'almond paste' is the definition.
(I've seen this before)
'tarnish one vessel containing unknown' is the wordplay.
'tarnish' becomes 'mar' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'vessel' becomes 'pan' (vessel or container for cooking).
'containing' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'mar'+'i'+'pan'='maripan'
'maripan' going around 'z' is 'MARZIPAN'.
(Other definitions for marzipan that I've seen before include "Paste on cake" , "Ground almond paste" , "Cake icing made of ground almonds and sugar" , "almond concoction" , "Marchpane" .)