Material from sewer in sea water? (5)
I believe the answer is:
rinse
'material' is the definition.
(rinse is a kind of material)
'from sewer in sea water?' is the wordplay.
'from' indicates a hidden word.
'RINSE' is hidden within 'sewer in sea water'.
(Other definitions for rinse that I've seen before include "Hair colourant" , "Run water through" , "Hair tint" , "Cleanse in water" , "Wash in clean water" .)