Japanese mat (6)
I believe the answer is:
'japanese mat' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for tatami that I've seen before include "Jap. floor mat" , "Floor-covering" , "Japanese straw mat; a mat, it (anag.)" , "Jap. room matting" , "Japanese rush and straw matting" .)