Sponge John having picked up a bit of fluff (5)
I believe the answer is:
loofa
'sponge' is the definition.
(I've seen this before)
'john having picked up a bit of fluff' is the wordplay.
'john' becomes 'loo' (both can mean a toilet).
'having' means one lot of letters go next to another.
'picked up' says the letters should be written in reverse.
'bit of' says to take the initial letters.
The initial letter of 'fluff' is 'f'.
'a'+'f'='af'
'af' reversed gives 'fa'.
'loo'+'fa'='LOOFA'
(Other definitions for loofa that I've seen before include "scrubber from Bath" , "Something for the bathroom" , "Fruit used for a sponge" , "Gourd used as a sponge" , "clean-up with this" .)