An idiot backed hard bathroom sponge (6)
I believe the answer is:
loofah
'bathroom sponge' is the definition.
(I've seen this before)
'an idiot backed hard' is the wordplay.
'an' becomes 'a'.
'idiot' becomes 'fool' (I've seen this before**).
'backed' shows that the letters should be reversed in order.
'hard' becomes 'h'.
'a'+'fool'='afool'
'afool' written backwards gives 'loofa'.
'loofa'+'h'='LOOFAH'
(Other definitions for loofah that I've seen before include "Fibrous part of gourd used as scrubber" , "tropical plant" , "Halo of the washing sponge?" , "Fibre used as a back-scrub" , "Dry sponge used for scrubbing" .)