Simpleton sent back a scrubber (5)
I believe the answer is:
loofa
'scrubber' is the definition.
(I've seen this in another clue)
'simpleton sent back a' is the wordplay.
'simpleton' becomes 'fool' (fool is a kind of simpleton).
'sent back' is a reversal indicator.
'fool' back-to-front is 'loof'.
'loof'+'a'='LOOFA'
(Other definitions for loofa that I've seen before include "Gourd used as a sponge" , "Bathroom sponge" , "cleaner" , "scrubber from Bath" , "American scrubber" .)