Good-bye! I am returning a Japanese mat (6)
I believe the answer is:
tatami
'a japanese mat' is the definition.
(I've seen this in another clue)
'good-bye i am returning' is the wordplay.
'good-bye' becomes 'ta-ta' (I've seen this in another clue).
'i am' becomes 'I'm' (contraction).
'returning' says the letters should be written backwards.
'im' reversed gives 'mi'.
'tata'+'mi'='TATAMI'
(Other definitions for tatami that I've seen before include "Japanese rush and straw matting" , "Jap. room matting" , "Floor-covering" , "Japanese straw mat; a mat, it (anag.)" , "Jap. floor mat" .)