Japanese horseradish (6)
I believe the answer is:
wasabi
'japanese horseradish' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for wasabi that I've seen before include "Flavouring used in Japanese cuisine" , "Pungent green paste used in Japanese cuisine" , "Hot Japanese paste" , "Herb" , "Pungent green root used in Japanese cookery" .)