Fibre used as a back-scrub (6)
I believe the answer is:
loofah
'fibre used as a back-scrub' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for loofah that I've seen before include "Dried pod commonly used as bathroom sponge" , "Vegetable bath sponge" , "Dry sponge used for scrubbing" , "Fibre used as a back-scrub" , "Bath scrubber" .)