Sponge ass's back — pleased expression (6)
I believe the answer is:
loofah
'sponge' is the definition.
(I've seen this before)
'ass's back pleased expression' is the wordplay.
'ass' becomes 'fool' (both can mean a stupid person).
'back' says the letters should be written in reverse.
'pleased expression' becomes 'ah' (expression of satisfaction).
'fool' written backwards gives 'loof'.
'loof'+'ah'='LOOFAH'
(Other definitions for loofah that I've seen before include "Dried part of plant used as a bath sponge" , "cleaner" , "Fibre used as a bath-scrub" , "climber" , "tropical plant" .)