Pastrami sometimes contains paste used in Japanese cookery (4)
I believe the answer is:
miso
'paste used in japanese cookery' is the definition.
'pastrami sometimes contains' is the wordplay.
'contains' says the answer is hidden in the clue.
'MISO' can be found hidden inside 'pastrami sometimes'.
(Other definitions for miso that I've seen before include "Thick paste used in Japanese soups and sauces" , "Soya bean paste" , "Soya-beam paste" .)