Bum in boozer (6)
I believe the answer is:
sponge
I believe this is a double definition.
'bum' is the first definition.
('sponge' can be a synonym of 'bum')
'boozer' is the second definition.
(thesaurus)
'in' is the link.
(Other definitions for sponge that I've seen before include "Light cake; sea creature" , "Bathroom scrubber" , "Sea animal often found in bathroom" , "Porous mass in oceans" , "Marine creature - cadge" .)