Bum in boozer (6)
I believe the answer is:
Here is my best explanation:
I believe this is a double definition.
'bum' is the first definition.
('sponge' can be a synonym of 'bum')
'boozer' is the second definition.
'in' is the link.
(Other definitions for sponge that I've seen before include "Piece of porous rubber", "that lives in the sea", "Marine creature - cadge", "Used for washing", "Porous substance to absorb liquid", "Aquatic invertebrate -- type of cake", "Sea animal often found in bath", "Drunkard", "Porous mass in oceans" and "Cadger".)