Rubbish in dirty water (5)
I believe the answer is:
bilge
This clue might be a double definition.
'rubbish' is the first definition.
(I've seen this before)
'dirty water' is the second definition.
'bilge' can be an answer for 'water' (bilge is a kind of water). I am not certain of the 'dirty' bit.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for bilge that I've seen before include "Filthy water that collects in the bottom of a boat" , "Lower part of hull; nonsense" , "Very long film, book etc" , "Nonsense (informal)" , "Rubbish, nonsense; dirty water in ship" .)